A Gang is a community of players who join forces so they can communicate with each other, test setups in friendly fights and participate in the upcoming gang battles. Gangs open after you finish 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 the Gang of your friend in "Friends" section or create your own Gang in "My Gang". There can be up to 25 players in one Gang. Creating a Gang costs 100 gems.