Generally, mobs are found in small groups. If you killl a few of that group, they won't respawn till you kill the rest of it. So, f.e., if you have 4 security guards in a building lobby, and kill 3 of them, everyone will see only one guard at this place till he's been killed, too. Then all four of them will respawn after a short time.
Because the gang bosses are always "members" of such groups, if someone kills the boss and leaves the others behind, other players have to kill all the gang members around and then wait for them all to respawn for a couple of minutes. If the coords are right, the boss will be among them.
Shortly, just kill all ASPs around the center of the roof and the boss should appear. If he doesn't drop the key (which often happens), just kill the mobs around again and so on.
I hope I could help you.