Gang Box is the box that you can only get when you’re in the Gang. During 3 days all members of the gang earn trophies in 3v3 fights to open the Gang box.
When the timer ends, you can open the Gang box and get rewards. Rewards' quality depends on how many trophies you collected with your gang mates. You need to earn the minimum number of trophies in order to get the Gang box. You can get any rewards from a gang box, even the parts that you haven't unlocked yet.