1
general::
1. Also, gang up with. Act together as a group. For example, The residents ganged up to make the neighborhood safer. [Colloquial; c. 1920] 2. gang up on or against. Join in opposition or attack against, as in The big kids were always ganging up on the little ones, or They all ganged up against the substitute teacher. [1920s]
American Heritage Idioms
2
general::
Phrase(s): gang up (on someone)
to form into a group and attack someone. (Usually a physical attack, but it can also be a verbal attack.) • We can’t win against the robber unless we gang up on him. • All right, you guys, don’t gang up on me. Play fair!
McGrawhill's American Idioms And Phrasal Verbs