Time for Gordon to play some golf, apparently. Before he can hit the links, though, you'll have to fight your way through a zombie roadblock. Kill off everything here, check out the room on the right side of the tunnel (accessible through a small vent), then use your grav gun to clear the cars off to the side and proceed. You'll come to a house soon enough, controlled only by a couple of Overwatch sentries. There's nothing of particular note in the house, save some crossbow ammo on the top floor; as soon as you get this last, though, you'll have to run from five or six rolling mines that drop from the roof. If you get badly hurt, check behind the beached ship nearby for a couple of medpacks. This is all optional, of course; if you just want to be on your way, keep following the road and bypass the house altogether.
Your first big challenge here comes in the form of an Overwatch checkpoint. There isn't anything too taxing within, though. If you attempt to rush through it, your car will likely be overturned by ramps that spring up from the road, so dismount before you hit the first gate, approach the town from the side, kill everyone within, then take your car through as smoothly as you please. There are half a dozen Overwatch inside, but you should be able to handle them.
The next ghost town is initially impassible; there's a gate blocking the way with no obvious lever or handle to manipulate. What you have to do here (after killing off the Overwatch that land, naturally) is find a couple of batteries and insert them into the machine that's powering the gate. There's only one building here, and the machine is inside it, so that shouldn't be difficult to find, but locating the batteries can be trouble. There's one in a blue-grey car up on cinderblocks outside the building (look in the engine block; it's easier to lift this out by hand than to use the gravity gun), and another one underneath an overturned bathtub on the otherside of the building; use your grav gun to push the tub off of it. When you insert both of them into the machine, the gate will rise, letting you pass through.
Not Enough Gunship
Time to say goodbye to your buggy, because you'll be ditching it at the next outpost you reach; this one, luckily, is filled with underground troops who're about to fend off an Overwatch assault. You'll have to help, so save your game and make sure your shotgun is fully loaded.
The Overwatch troops come in in a series of four dropships which'll unload troops on each side of town, but they only come in one at a time, leaving you with only four or five soldiers to deal with at a time. Your squadmates will do a decent job of giving them secondary targets, especially if you hole up inside one of the buildings and force the Overwatch to come to you. Be sure to stay away from windows, no matter how tempting the prospect of sniping the Overwatch is; the dropships are well-armed with turrets, and will blast you they get you in their sights. You should be able to find plenty of ammo on the Overwatch and in supply crates, especially in the largest building, so this essentially becomes a game of waiting them out. If you prefer to work alone, you can run up to the lighthouse on the cliffside edge of town and go in there; your teammates will likely have a hard time following you, but this makes it a bit easier to kill the Overwatch as they arrive, since there's only one door.
After the fourth dropship (which lands at the lighthouse), a gunship will appear. The only source of infinite rocket ammo is a box on one of the upper levels of the lighthouse, so head upstairs and get bucked down! The gunship, as per usual, isn't too horrifically difficult to take out; just shoot your rockets straight up from the stairs before heading up to the roof, then guide them back to the gunship as it attempts to shoot them down. Check our video if you're having trouble!
When the gunship's destroyed, you'll be escorted to a secret cliffside path that's located in the basement of the lighthouse. Take the path through a dark tunnel to reach the next loading screen.