You can't give everyone as much ammo as they got in TF2 because there are fundamental differences between these two games.
First, the weapons in TF2 deal a lot more damage than in gg2. For example a TF2 scout can kill a heavy in three Point-Blanc shots and everyone else with two shots. The runner needs five shots for an overweight, and three for most other classes.
Secondly, there are no secondary or melee weapons in gg2. This means when you're playing gg2 and your primary is out of ammo you're pretty helpless. In TF2 on the other hand you can switch to your secondary and use it to kill at least one other enemy, then you can take out your melee weapon and the melee weapon never runs out of ammo. You can kill enemies with it as long as you stay alive.
So yeah, if you look at this, then it should be clear, that all classes need more ammo.