Stats
The Gunzerker is a STR/DEX/CON based subclass that closely resembles a Barbarian Berserker but also has some unique abilities.

Proficiency
The Gunzerker shares the same stat priorities and proficiencies as a Barbarian Berserker. The Guzerker should never use any weapon other than the Gunzerker Gunblade which is a 2h greatsword.

Unique Gunzerker Weapon, The Gunzerker Gunblade
The Gunzerker uses a special greatsword with firearm parts attached to it, this weapon is called a Gunblade.  Most Gunzerker ranged attacks are limited to 9m. This is a tradeoff since having a gun attached to a greatsword would make it rather unwieldy and inaccurate. Also the ammunition is quite large in order to facilitate the special shots, think of them as being almost the size of a shotgun shell.  The starting Gunblade will level with you and take you all the way to end game. Alternately, you can use the forge system to combine existing vanilla game weapons with a Gunblade template. More about the forge in this article.

Ka
This is a class resource that all Gunslingers use to power some of their abilities.

Gunblade Mechanics / Overheat
The Gunblade grants you the OVERHEAT mechanic. Any time you deal burning or fire damage you gain 2 stacks of Overheat. You can carry a maximum of 20 stacks of Overheat.  You lose overheat at the rate of 1 stack per turn.

While overheating Gunzerkers take 1d4 fire damage per turn. They will also do AoE fire damage of 1d4/turn to all enemies within 3m of them. Gunzerkers cannot be targeted by attacks of opportunity while overheating.

When you activate the Overheat Convergence passive toggle (on the far right of your hotbar) the next attack you do that deals fire damage will also deal bonus fire damage equal to the amount of stacks of Overheat x 2.

Progression

The Gunzerker gets all the abilities of a Barbarian Berserker plus the following.

Level 1

• Flame Strike - a melee strike that does weapon damage plus scaling fire damage. Does extra scaling fire damage when used with Overheat Convergence.
• Fire Blast - a short range cone fire attack that does half weapon damage as fire damage to all creatures in the cone.
• Gunblade Slug - Fires a red hot high velocity round in a 9m long line. Each creature in that line must make a Dexterity Saving Throw to avoid taking full damage.
  

Level 3

• Fire Leap - Only available while raging. Jump in a straight line to an empty space of your choice on the ground within 60 feet. When you land, the blistering air blasts out from you, dealing 1d4 bludgeoning damage and 1d6 fire damage to all creatures within 3m that fail a Dexterity saving throw. Those that save take half damage.
  

Level 6

• Firebrand - Your abilities ignore Resistance to fire damage. In addition, when you deal Fire damage, you cannot roll a 1.
  

Level 8

• Improved Fire Leap - Can now be used even outside of rage.
  

While the unique kit may be small, the additional abilities granted by the Barbarian Berserker progression more than makes up for it.