Ganking is a gaming and slang term that usually means a surprise, often unfair, attack or takedown.
In gaming
In online games (especially MOBAs and MMOs), “to gank” means to ambush an enemy player, often by outnumbering them or catching them when they have little chance to fight back. This might be, for example, two or three players suddenly jumping on a single, weaker or distracted player to quickly kill them.
In general slang
Outside games, particularly in US and Canadian slang, “to gank” can mean to rob, cheat, or steal something from someone. People might say they “ganked” an item to mean they took it, often sneakily or dishonestly.