Gebbing is the act of scratching one's anus with a finger. Typically done to determine the amount of feces, sweat, or other slime that has accumulated around one's anus. Also, it feels really fucking good. Of course, if you use toilet paper, then it's no longer gebbing, it's just regular wiping.
Gibbing, deriving from the word "giblets" meaning the offal of a dead animal, is a video gaming slang term featuring prominently in many 3D shooter games.
The word refers instances when a game character has been killed with such force that their body explodes leaving their "giblets" visible all over the floor.