Douglas Harper's Etymology Dictionary
by 2000 as the verb that indicates the situation of many players or NPCs simultaneously attacking one; gamer slang, perhaps borrowed from hip-hop and drug-abuse slang (where it is attested by 1995 in the sense of "to rob, to rip off"); perhaps by 1990 in sports jargon. Of unknown origin; perhaps ultimately based on gang (v.). Related: Ganked; ganking.
Wiktionary
vb. 1 (context transitive slang English) To swindle. 2 (context transitive slang English) To steal. 3 (context transitive Internet online gaming slang English) To kill, ambush, or defeat with little effort; used in online games.
Usage examples of "gank".
The broad-shouldered Gank lifted the spiders and tossed them against the walls.
Jabba before he sends any more of his remotes, Gank killers, and who knows what else.
The three were sitting in the smoky shadows of the Mos Eisley Cantina, sipping green Pica Thundercloud and watching the bounty hunters drift in from around the galaxy: Weequays, Aqualish, Arcona, Defels, Kauronians, Fneebs, Quill-heads, Bomodons, Alpheridians - and the inevitable Ganks.
That was the main reason why, when the ganks administered TM, they seldom used physical/electronic lie detectors as auxiliaries.