I used to make all my own fish foods and never heard of using gelatin in them. I would take frozen seafood and beef hearts, and mix them in the blender with vegetables like peas or spinach, add any vitamin supplements I wanted in there, and then freeze it in serving size spoonfuls on a cookie sheet. I would then drop the frozen "fish cookies" (as my mom liked to call them lol) into the tank and let them thaw in the water. They would fall apart as they thawed, and the fish would gobble them.
I would not use flavored Jello. The sugar and colorings would pollute your water and be bad for your fish. If sugar and artificial colorings are bad for people, they are probably way worse for fish. =)