To be beaten to death by a large black rubber dildo.
In the movie Lock Stock and Two smoking barrels, the character Hatchet Harry, beats a man, suspected of stealing, to death with what happened to be at hand. Harry, also a porn shop owner, happened to grab a big black rubber cock.
This term arose due to the unwieldlyness of the lengthy phrase "He was beaten to death with a large black rubber cock..." and was shortened to simply, "He got gonked."
Hatchet Harry gonked some bloke for stealing from him.
If these guys keep pissing us off like this, I'm gonna gonk them.