At Level 1 you will want Scrap-o-Matic Smelter this will increase your mana return from salvage and extends your time out in the field. This is very important because Gazlowe has major mana issues without this talent and you will find yourself porting back frequently which will waste a lot of time.

Mercenary Lord at Level 4 will reduce the damage you take from all minions and mercs. This is very important to this build since your party will depend on you for mercenaries and later the boss mob if you find the opportunity.

Most mercenary camps have multiple targets. So having Rock-It! Turret XL is a good idea. This allows you to also fire at multiple hero targets when not at mercenary camps making you a greater threat. Additionally when taking down structures the turrets will target multiple structures/minions in their range as well allowing you to siege faster.

At Level 13 you get Turret Storage which gives you an extra turret. At this point you can actually solo a golem. Forget what the ignorant non-Gazlowes sometimes say about him being able to solo it as soon as he gets Robo-Goblin at Level 10. That isn't the slightest bit true and they would know that if they played Gazlowe. You can solo the boss camp after you get your extra turrets, you will need the extra turret dps though and the HP from the extra levels.

Level 16 will get you Kwik Release Charge. This helps with merc camps, golems, and in general making life miserable for the enemy team since you can throw a bomb out and then throw another. An alternative which I find very helpful is Stoneskin which will give you more survival in fights.

Level 10 & 20
Your ultimate will be Robo-Goblin for this build because it gives you heavy damage to mercenaries and structures. At 20 you will want to take Hardened Shield to further reduce damage taken. This will further help with taking golem boss camps and surviving group fights.

At Level 1 you will want Scrap-o-Matic Smelter this will increase your mana return from salvage and extends your time out in the field. This is very important because Gazlowe has major mana issues without this talent and you will find yourself porting back frequently which will waste a lot of time.

Level 4 you will want to pick Goblin Fusion to power up your lazer damage. If you aren't use to using the lazer it can take getting use to using it but once you get down the when to charge up and fire then it is a powerful tool that is often overlooked by less experienced Gazlowe.

Rock-It! Turret XL at Level 7 provides you with multiple shots to your turrets helping in group fights and mercenary camps. Optionally Engine Gunk provides a slow which can help catch enemies if and if you have a pre-set team that talks things out you may find this extremely useful with setups that include the Executioner talent.

Level 13 is the EZ-PZ Dimensional Ripper which allows you to temporarily lock down structures and slow minions with your lazer. This helps with lane pushing. Optionally you can go for Turret Storage for one extra turret which will give you an extra one that slows targets or if you chose XL turrets even more pushing.

Hyperfocus Coils at Level 16 is a must for this build. This decreases charge time of the lazer!

Level 10 & 20
Your ult for this build is going to be Grav-O-Bomb 3000 with it's upgrade Miniature Black Hole.