From the docks, you are going to want to head north east a little bit and locate the guard tower. Use the rooftops or the ladder to climb this and eliminate the sniper here. From this guard tower you should be able to find and destroy the first two barrels of poison gas. The first is by the small shed to the north and the other is by an open storage area to the west - note that you’ll need the sniper’s rifle to shoot this one from the tower so if he drops it off the tower when you kill him then you are going to have to unfortunately get in range to shoot it!
The final barrel, the flag and the leader are all located inside the walled fort-type area on the western side of the Lac Eternel map. Fortunately, there is a not so subtle stealthy way into the area via the exposed scaffolding to the left of the main gate. Climb this to the top of the wall.
Follow the wall to the left and drop down to ground level to find the final barrel of poison gas by some guards and British prisoners. Destroy this and then stealth your way back towards the main gate and activate Eagle vision as you go to locate the leader. Kill him when you get the chance.
Finally, climb up and interact with the flag to complete the takeover.