Originally from the Sony Online Entertainment multiplayer online
game launched in 1997 and available until 2010 called Tanarus. It was a
game of 6 players per side in various tanks of their choosing, fighting in complex areas for either total annihilation, points or capture the flag. Players could communicate with their team and when one
player was isolated and was attacked by more than one tank they would shout in group chat for example "help I am being
Gang Tanked in sector one."
Gang Tanked later evolved into "Ganked" as it was easier to type fast i.e. "Ganked sec 1!" It is now synonymous with being attacked by two or more when alone in a MMPORPG.