An amazingly fun mod for the Counter-Strike franchise. You start off with the crappy glock and each kill gets you a level up which gives you the next best gun in line. The order varies per server. There are different variations and rules such as knifing steals a level from your opponent, afk kills do not count, TKing means you level down and so on. You can only level up using the current gun with your level that way you can't pick up a gun from a person 4 levels ahead of you and go on a rampage. You can get as many levels per round until you die, my record is 6 levels per round. Grenades are the hardest to level with since you always have friendly fire on when someone is on the grenade level.
Gun game is the only reason I play CS anymore.