The Gangs of Novigrad is a side quest in Novigrad.

This quest is only available during the main quest “Get Junior.” It is available after the bathhouse encounter. It can be failed if you do not see the dwarf Cleaver before going to see King Radovid.

There are a few ways to clear out the casino and the arena without Cleaver's help so if you want to complete this quest make sure you go see Cleaver before doing anything. All you have to do is agree to help the dwarf’s men kill all of Whoreson Junior’s men at his casino and arena.

It folds in perfectly with the main quest, since you have to clear those establishments anyway. Check out the page for “Get Junior” for more detail on these specific battles.