Only Custom Mode Works in this game. Or You can play Other Versions; Gun Mayhem 2 and Gun Mayhem 3

Gun Mayhem 1 Unblocked

Control Keys: 

Arrow Key to Move and Jump

Shoot with key

Bomb with key

Fight until you fall down, but remember you get a limited number of lives and you need to make your opponent fall if you want to win. This way you can win Gun Mayhem game.