Comes from my days of playing Quake and similar games. When a player gets "blowed up real good", there is an explosion of blood, guts and body parts flying in every direction. In game parlance, the body parts are "giblets" - or "gibs" for short.

I'll let you guess whether Mr-Gibs indicates my skill at reducing other players to gibs, or my lack of skill and resulting numerous messy deaths. ;)