This gang headquarters takeover is extremely easy thanks to an abundance of stalking zones and rooftops to utilise and not so many enemies wandering about. If you have unlocked the viewpoint in the area for fast travel, I would recommend starting the takeover from there as it is pretty central and the leader patrols right beneath it.
Drop down to the main roof of the viewpoint’s building and head to the northern corner. Switch on your eagle vision to locate the leader and then perform an air assassination on him as he passes by. After killing him, quickly run for the closest poison gas supply and jump into a nearby stalking zone – there is actually one inside the undercover area where it is located – just for convenience! Shoot it from here.
Now we are going to want to sneak over to the next set of barrels nearby to destroy the second poison gas store. Finally, make your way over to the large wooden wall in the north-western part of the area before proceeding to climb it, killing the sniper here and approaching the flag to cut it down.