'Gank' has two main slang senses. In video games (especially MOBAs like League of Legends or Dota), to 'gank' means to ambush an opponent with a surprise attack from another lane — standard gamer vocabulary, harmless. In older street slang and 1990s hip-hop, 'gank' also meant to steal or cheat someone. If your teenage gamer says they were 'ganked', they almost certainly mean the in-game ambush. Context usually makes it obvious — gaming chat vs. real-life chat. No need for concern in the gaming sense; ask a gentle question if it appears outside a gaming context.