Using Gelatin as Rocket Fuel?
Powdered sugar is often used in chemistry demonstrations, in the presence of oxygen it is highly combustable (there have been significant explosions and fires at sugar processing plants in the past due to this).

The sugar is high in energy, and the fine powdered variety has a greater surface area with which to interact with oxygen, producing the burst of energy needed. The sugar by itself however, is probably not enough to get any propellant effect, you'd need to add some kind of oxygen source. A quick google suggests mixtures of powdered sugar and potassium nitrate (link). 

Dry gelatine (the powdered version) is essentially just sugar plus, as you noted, animal extracts or vegetarian friendly alternatives, so the same should apply, though using actual sugar is probably a lot cheaper. 

Edit:  As pointed out by @MaxW in the comments below, and as eluded to in my opening comment, igniting a fuel in the presence of an oxygen source can be highly explosive. Be sure to take necessary safety precautions (work on a small scale, away from buildings. Ignite from a distance, and be sure any spectators are also out of harms way etc.) and also ensure that you're not breaking any laws in your area.