Gunner class is mainly anti tank and damage dealer. The Gunner can eat grenade or explosives throw at it. In Battlefield Heroes Gunner is the slowest speed class but with 150 hp and big guns, can stand ground and defend flags.
Gunner class has the larges amount of ammunition but they are good only in short and medium range.
Frenzy fire can replenish life when damage the enemies, and keg can slow down enemy commandos long enough to be an easy kill.
As soldiers team playing is require from gunner class, getting a few gunners with a medic is the best attack party, and result in wining the map.
Because of the big damage output, the gunner class is the easiest to level up, getting a lot of point per map.
Commandos can be easily kill when they try to knife a gunner, buy using Leg It and gunning it down, Keg is a good option too, especially on Midnight Mayhem slowing enemy down and reveal enemy commandos for a short time.