Short for Game Killer. A similar, more masculine form of co*k-block, a GK waits until things are going well and proceeds to irreversibly destroy a friend's ability to get ass. This is usually achieved by trying so hard that it makes everyone uncomfortable.
I was making serious headway with a Bulgarian (I think) skinny blonde with a ginormous rack when GK suddenly appears in my peripheral. After thirty seconds awkward conversation and bad jokes, the blonde leaves.