A gang is a community of players who join forces so that they can communicate with each other, test setups in friendly fights, and participate in the upcoming gang battles. Gangs open after you complete the tutorial.
Gangs can be found by tapping on the group icon in the main menu. You can find a particular gang using a search function, join your friend's gang in the "Friends" section, or create your own gang in "My Gang". There can be up to 25 players in a gang. It costs 100 gems to create a gang.