1. What does it mean?
갱 is short for 갱킹, ganking. In games such as MOBAs, it means moving in unexpectedly to help an ally pressure or defeat an opponent, often by creating a temporary numbers advantage.
Also written or said as 갱킹, gaeng
2. The nuance
This is game vocabulary, not a real-world gang. A 갱 may succeed by forcing an opponent back even if nobody is defeated.
3. Examples
“정글에서 갱이 올 수 있으니 주변을 봐.”
Watch the area because a gank could come from the jungle.
“갱 덕분에 밀리던 라인을 다시 정리했어.”
The gank helped us stabilize the lane we'd been losing.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.