Approach this headquarters from the east, climb to the rooftops and eliminate the sniper wandering about here. From the rooftop with the sniper, if you look down whilst facing off the roof to the north with eagle vision active, you should be able to spot the gang leader as he patrols by. Drop down ad assassinate him when you see an opening to do so.
Keep moving along the rooftops and take out the gang member guarding the flag before cutting it down to complete objective number two. Afterwards, we need to find the British traitor, who conveniently enough appears to be the only enemy on the deck of the nearby ship. Simply run over there and take him down any way you see fit.
Once all three objectives are complete the gang headquarters now belongs to you!