I can think of a few ways of set up, but it depend on devs to decide if they want add them:
1.Add a new turf location and every time some gang caputure it, just spawn a mobile HQ similar to the cops one but for the gang that control the turf? And when the turf is controled by other gang, just despawn it?
2. just sell them at rebel outpost, one time purchase, can not put back to garage. And extend it's despawn timer.