The best thing to do is make a pulley system. The one I made is fairly simple. The reason you would want to make a pulley system is that you can lift more weight with less work.
In a system of a single rope and pulleys, when friction is neglected, the mechanical advantage gained can be calculated by counting the number of rope lengths exerting force on the load. Since the tension in each rope length is equal to the force exerted on the free end of the rope, the mechanical advantage is simply equal to the number of ropes pulling on the load. For example, in the diagram below, there is one rope attached to the load, and 2 rope lengths extending from the pulley attached to the load, for a total of 3 ropes supporting it. If the force applied to the free end of the rope is 10 lb, each of these rope lengths will exert a force of 10 lb. on the load, for a total of 30 lb. So the mechanical advantage is 3.
In my system, there are only 2 rope lengths attached to the load so the mechanical advantage is 2. In other words, I will only have to pull 50 pounds to lift a 100 pound deer.
Here is your shopping list:
2 pulleys rated to at least 125 pounds
rope
2 screws with eyes on the end to support the system
2 carabiners
1 gambrel
The other thing that would be nice is a cleat to tie your rope off.
Just make sure to bring a drill to drill pilot holes for the eye screws.