In Homestuck, the Prankster's Gambit is a measure of a player's one-upmanship over other players in the Homestuck Omniverse, as well as the name of the Egbert family joke shop. Prankster's Gambit cannot be recovered or replenished by items found on slain monsters; it can only be exchanged with other players in a 'tit-for-tat' manner. Depending on the scale of the joke or prank, points are withdrawn from the victim's Prankster's Gambit total, and added to the total of the person who committed the prank. The points lost by the victim have thus far proven to be equal to the points gained by the prankster.
There have been a few characters who've been seen with the Gambit's meters: John Egbert, Dad, Nanna, Rose Lalonde, and Jane Crocker. John lost points to both Dad and Nanna early in the game but most likely reclaimed them after pranking Rose. Rose's meter hit the lowest point possible when she realized she just can't get back at him for it. John's gambit meter in particular hit rock bottom after a round of repeated pranking by Nannasprite.
Calliope and Caliborn were shown to have a Prankster's Gambit during their chess game. Oddly enough, the gambit seemed to switch between red and green, which foreshadowed the fact that Calliope and Caliborn shared a body.
Prankster's Gambit has, thus far, shown no useful purpose.