You would have killed some snipers through an upstairs window on the left hand side of the street. Go up the stairs into the room (it is to the left of the gate) and pick up all the stuff that is on the floor. Wait until you pick up the bazooka and then the next objective becomes active.
Then go an try to open the gate. I will stop there so as to not ruin your game. But you should be able to get on with the game now.