I'm just a guy, nothing special about me. I've been playing Gall with my roommmate basically non-stop since this two-headed hero came out, and we're not only having a total blast, but we're owning quickmatch with about a 75% win-rate in just under 50 games played. I've noticed that there aren't really any good guides out for him right now, probably because no one is taking him seriously, so I figured I would step in and put something out there.
I'm writing this guide assuming you have someone that you know well playing with you, since communication is key in winning with Cho'Gall. If you're not actively talking to your Cho, at least work out a set of rules or guidelines before-hand, and do your best to stick to them.
Gall is very very easy to play, but has a much higher skill cap than you think. Because you don't ever have to concern yourself with movement, you should constantly be looking around the map to see which enemies are where, check if they're going for the boss, which camps are up. Think of Cho as the "Driver" and yourself as the "Navigator". If you're a less skilled player like myself, you can start practicing some of the high-level things like keeping track of enemy heroic cooldowns and talent picks, and pass this on to your team. If someone is flaming at an AFK Abathur, you've got the free time to tell them what's really going on. Don't be lazy just because you can't move! The other mindset you could have is the Gatling gun on one of those Vietnam helicopters (especially once you get Shadow Bolt Volley!). All of the damage is coming from you, but you need to intuitively know where your pilot wants you to direct that damage to keep you guys in the air.

All talents have no mana cost, and super low cool-downs. Fire away!

Q = Shadowflame
This essentially replaces your auto-attack, with something that does a lot of piercing damage for its 3 second cooldown. Use this constantly to poke into bushes, shoot Zagara's creep, and shoot any shimmers you see if there's a Zeratul or Nova on their team. Obviously try to line up targets. Not much else to say about this one; simple but effective.

W = Dread Orb (I call it Bouncy Ball)
This talent really deserves more credit. It bounces three times, but is maddeningly slow. You can usually hit something reliably on the first bounce, but bounce two and three are lost causes if you're aiming at a moving target. In a team-fight, they disrupt enemy positioning, but you're basically just hoping they do something. When laning or getting camps, these will kill minion waves super well (you can land them close together to wipe out a wave completely), and you can snipe towers with them too, since this is your longest ranged ability (outside Shadow Bolt Volley). The least obvious application is chasing heroes. The bounces move at basically the speed that a hero does in a straight line (unmounted). If that Raynor is trying to run from you, and turns to shoot for even a second, you can start the bouncy balls comin'. If he keeps going in a straight line, he gets three balls to the face, and if he doesn't, he can't escape as fast. It's a very versatile tool.

E = Runic Blast (Bowling Ball)
This is by far your strongest talent. The ball by itself does basically nothing, but when you pop it, it can melt through groups of heroes, and is perfectly sized to kill a minion wave. The worst thing you can do is miss it (since it's your longest cooldown, and Cho might even stop rolling them if you're never popping them), and always be looking out for your Cho's ball animation. When I play, my Cho will say "ball" (yes very complicated) when he rolls one out and wants it detonated immediately. If you can get this down, Cho will never need the super useless "roll-back" talent, and can get something better. He'll often tell me who he's targeting if it's someone specific. This ability can really separate the good and bad Cho'Gall partners.

Z = Hurry Up, Oaf!
Don't forget about this!!! It should be only used for escaping and chasing. If Cho is outside of combat, he should be mounting up, not getting a temporary (and rather weak) speed boost. The decay over time is very quick, and the first half-second is where most of the speed comes from. If you have excellent Cho-Gall communication, let Cho decide when to use it by having him call out "Speed boost" or something. If Cho is getting stunned, wait for the stuns to pass before you use this, or you'll have wasted it.
EDIT: With the talent being freed up at level 1, it becomes easier for both you and Cho to buff this. If Cho takes the "No decay" talent, and you take Hurry Up Oaf!, you become hilariously fast for a ridiculous amount of time. It also happens to be super useful, so give it a try!

Heroics: Shadow Bolt Volley and Twisting Nether
I'll talk about these later more, but, unless it gets buffed, I can't think of a reason to ever get Twisting Nether. EDIT: Unless it gets buffed even more, Twisting Nether is still far inferior.

Level 1:
Eye of Kilrogg is a fun and pretty useful talent, and I've heard strong arguments for it for preventing ganks. The logic is that you represent two kills for the enemy team, and will be targeted more often. While true, I find the constant uptime on Z provided by Spurred Onward to be more useful in general.
Spurred Onward: With this talent, you can use Hurry Up, Oaf! basically whenever you'd like. Secure kills, escape, or just for general movement. Fun fact: if you use this, cast Shadow Bolt Volley, and hit 20 enemies (very easy at level 20), the cooldown on Z is instantly reset.
Shove is not good enough to be a dodging tool, and will only mess up what your Cho is doing for positioning. Don't annoy him with this useless talent, when you could be doing literally anything else.

Level 4:
Bomb's Away is the best talent of this ''meh'' tier. Is extra range useful? Sort of, but if there were anything better you bet I would take it in a heartbeat. The thing is, Runic Persistence does essentially nothing since the ball's base damage is worthless, even with Rollback. And I have a mini-rant about Double Back that you can skip if you don't care.
I think Double Back is one of the most useless talents Gall has. Arguments made for Double Back are: 1) it makes it easier to hit the third bounce, 2) all three bounces hit towers when sieging, and 3) synergizes well with Rising Dread at 13 because the third bounce hits better.
Here's my experience: The third bounce will never ever hit a moving target. It take several seconds for it to land, which is longer than almost any stun in the game. Give up on that dream. And if you're thinking it will be good against melee heroes, if you're trading AA's with a hero for that long without moving, you've got bigger problems on your hands. As for sieging, If Cho is melee hitting towers/forts (as he should) you can hit all three bounces on the tower without this talent. Try it sometime. I do it often. Works on minion waves too. When you're at longer ranges, this talent completely removes your ability to snipe towers/fountains/forts at extreme ranges. What this talent does is remove the range from your longest ranged ability, while adding useless utility. Not worth it in my opinion.

Level 7:
Dark Descent OR Edge of Madness: Now we're cooking. Here's your first real "damage tier". The choice is basically between Dark Descent and Edge of Madness, since Cho's Consuming Blaze effect is short and the cooldown is long enough that you will use Double Trouble rarely, although inst-casting 3 shadowflames in a row is pretty fun. So you have to choose between Dark Descent for a 20% bonus on your strongest ability that you have to build up again every time you die, or Edge of Madness for what will average out to be around a 10% buff to your most spammable ability? I've tried both, and both are good. One last thing to consider is if you'll be picking Giant Scorcher at 16, you'll probably want Edge of Madness to go with that.
The blanket advice I'd give with this one: if you're dying a lot, pick Edge of Madness. If you're consistently landing those bowling balls and living, pick Dark Descent.

Heroic Time!
Here's the math behind why you should always choose Shadow Bolt Volley over Twisting Nether (you've been warned): 20 shadow bolts x 108 damage/bolt = 2140 damage total. Now, this ability is very hard to aim, but has a short cooldown, so you're doing 2140 damage/minute * 60% (conservatively) accuracy = 1248 damage/minute on average.
Twisting Nether does 436 damage to everyone within its decently-sized circle every 100 seconds. Max damage (to heroes) you can do is: 436*5 = 2180 damage every 100 seconds = 1308 damage per minute, which is just barely more! And that's hitting all five heroes every time! I'm lucky if I hit 2 heroes with this thing! I've lost games because I was trying to make Twisting Nether work. You'd think it would synergize well Cho's Upheaval heroic, but Shadow Bolt Volley actually works better with it!
So pick Shadow Bolt Volley. If your Cho is trading AA's with a melee hero ( The Butcher comes to mind), point this machine gun lazer at him, and watch his health melt away. At level 10, the bolts are easy to not notice at close range, so he won't even notice his health disappearing half the time. Works great just to scare people off as they're chasing you too. Just point it behind you and let loose. This ability will melt anyone or anything every 60 seconds. The only trick is to communicate with Cho when you're going to fire. As your pilot, he needs to strafe the target properly to get a full blast. If you do this properly, you can hit your intended target with almost every shadow bolt. Cho will see a big purple arrow going down the direction you're about to let loose, but it's still helpful to call out who you're targeting if possible.

Level 13:
The decision is split between Dread Shield and Rising Dread. Survivability is good (the shield is surprisingly strong), but so is extra damage. "But dude, you've been saying the third bounce will never ever hit!" Yes this is true for moving targets, and especially if you're aiming for someone in particular. But anything that adds an average of 33% damage to your ability is fantastic, and great for sniping towers and forts. In a teamfight, there are worse talents to pick, since the bouncy balls will be flying everywhere, you're probably going to get in at least a few lucky hits with those second and third bounces. Remember that the shield will not proc on any enemy, it has to be a hero, so you have to choose between doing extra damage on your hardest to aim ability, or getting a decent shield off of the pretty reliable first hit and the potential for more if it hits again.

Level 16:
This is another flex-pick level, and you have to pick between Giant Scorcher if you're going against tanky teams (or even if there's just another Cho'gall on their side), or the Twilight Nova. I hear a lot of people saying the extra balls in the Nova just float around and look pretty. Not true. Mathematically, you're increasing your damage by 66%, which is insane (although the probability of missing goes up as well). In the chaos of battle, you're adding even more damage on your side, as well as opportunities to get a Dread Shield building up, or a juiced up Rising Dread shot in, depending on what you picked at 13. Your siege damage goes way up too, since that's a for sure 66% more damage if you're positioned correctly.
Shadowflare used to be my go-to choice at level one, but sadly it just can't compete with the other two at this tier. It's not a massive increase, and all it does is allow you to hit a whole minion wave (when they're single-file) if you're lined up right. It lets you hit more enemies more easily, and that's always a good thing. It's not a damage increase, but it's close, so if you really like it, go for it.


Level 20: Storm Tier!
Does Twisting Nether become the better ability taking into account the level 20 version of itself? Again, giving it the benefit of the doubt, it will at best receive a 50% increase in damage (I understand cc is important, but so is raw damage output). Shadowfury will make it so that you can hit as many targets as you want in a line, plus it makes it impossible to block this ability, making your accuracy go up. Going from 60 to at least 80% accuracy and (conservatively) saying you'll hit an average of 1.5 targets with this bumps up damage by a massive 99.5% at the very most conservative estimate. Piercing shadow bolts will ruin a clumped up team, since the tanks can no longer protect the back line, and you can not be stunned out of this ability. The numbers add up: Shadow Bolt Volley wins again.
Psychotic Break: *Sigh* Abilities/talents based around dying are never good, but one where you die, don't move, and only have enough time for 3 Shadowflames, one Dread Orb, and no bowling ball? There's some funny videos of Galls with this ability taking down a team that thinks the fight is over because they've killed everyone. They're extra funny because someone actually chose Psychotic Break over mother-effing piercing shadowbolts of death. Laugh at these people and move on with your life, never picking this talent ever. If you're getting focused all the time and dying, you're probably going to lose anyway, so go ahead and pick the talent if you want to try it out, then talk to your Cho about how he's got a passenger and should drive more responsibly. EDIT: Does 5% ability power sweeten the deal? Yes, of course, but it still can't outshine those piercing shadow bolts.

As a final note, if you go over the talents, you'll see that your late game is especially strong (this is reinforced if you look at hotslogs winrates with time). Early game, you're getting utility/survivability talents, and late game is when you start dealing it out like crazy. Tell your Cho this, so he knows your limits early on.

Remember, communication is key. Cho is the pilot, and you are the navigator/turret. All of Cho'Gall's damage is in your abilities, so you should consistently have the top stats for damage out of everyone. Work together, and you can have a ton of fun with this hero.
Let me know if you have feedback, especially if you're someone who is better at this game than me. This guide is written for quickmatch, so if someone wants to share their thoughts on how it applies to Hero League, feel free to do so!