My idea based off the game, The Godfather 2.
No set gang base for each gang... bare with me. No player owned businesses... bare with me. Each business can be taken over by a gang (like a party base). The gang that has taken over a business will collect the profits from that business while they have it occupied.
The businesses can be NPC run and stocked automatically at certain times, just like any other NPC ran business. The gang could have the option to stock certain items as they please. There could be NPC guys inside the business and their outfits can change depending on which gang has the business taken over.
All gang fighting can take place behind the counters, or in the main store to make buying items difficult

. A gang member putting his life on the line deserves reward, correct? The money earned from businesses can be divided among gang members in a paycheck form or they can be paid for how they perform in raids, etc.
If people want to make money they'll get on and raid... This would create constant raiding and actual competition for gangs.