Like when someone calls a big bet with a very marginal hand and sucks out on the river. Wp, says the opponent, but of course he means the opposite. GB and GC mean good bet and good call. From my experience these are used less sarcastically than wp . Gb and gc are more often genuine compliments.
CubGG operates within the sweepstakes rules where they are legal. The Site does not permit members to wager real money and has no ability to accept deposits as it is a "free play" site only.