The gang box is a box that you can only get if you’re in a gang. For 3 days, all members of the gang earn trophies in 3v3 fights to open the gang box.
When the timer runs out, you can open the gang box and receive rewards. The quality of the rewards depends on how many trophies you and your gang mates have earned. You need to earn the minimum number of trophies to get the gang box. You can get any reward from a gang box, even the parts that you haven't unlocked yet.