If you had played all the Gun Mayhem sequels, you will surely ask for more. There are a lot of shooting games which have similar features with Gun Mayhem and you should check this out. One worth mentioning is Fight for Glorton which is an action game where you will fight against the opponents; damage them until they are ejected out from the playing arena. You can also choose your character and you can opt to play in single or multiplayer mode.
Another game like Gun Mayhem is Superfighters which is an action game where you can play in single or multiplayer modes. In multiplayer modes, you can play as a team with the other player in the stage mode or you can play against the other player in Vs mode. Combat Hero Adventure is also similar to Gun Mayhem. It is an action adventure shooting game played in an arena.