What is meant by gang?

A group of people. You can talk about "the gang up here on the seventh floor" (meaning everyone who lives or works on the 7th floor) or "Joe and Florence and all the gang" (meaning their family) or "working on a chain gang" (meaning a group of prisoners chained together). Frequently it means a group of urban young people who form gangs as social peer support groups. Sometimes these gangs of young people are involved in crime.