This is where a block and tackle pulley system really comes in handy. You need some kind of very stout overhead limb or beam. I use a triple pulley system: a double wheel pulley at the top and a single at the bottom. The rope is tied to the gambrel, goes up over one top wheel, down under bottom pulley wheel (which is attached to the gambrel), back up over other top wheel, and then down to hands. This system divides the force needed to lift the game (equal to it's weight) by 3, so it only feels like you are pulling up a 100-lb. hog instead of a 300-lb. hog. You can add another wheel or two to this system; each additional wheel reduces the force by one more factor, i.e., 4 wheels divides by 4, 5 wheels by 5, etc.
You can also use a tree, a single pulley mounted to the front wall of a pickup bed, a length of stout rope, and a ramp firmly attached to the end of the tailgate that can slide across the ground to load a very large animal into the pickup bed. With the game animal lying beside a stout tree, tie one end of the rope to the tree. Back the pickup up to a spot where the game is close to the end of where the ramp will be. Put the ramp on the end of the tailgate, run the rope through the pulley, and tie the other end to the game animal. Then pull the pickup forward very slowly and the game animal will be pulled up the ramp and into the pickup bed.