Can I safely use gelatin to mix with 25% - 35% peroxide to make a gel or should I use glycerin?
That depends. There is no way to handle 30%+ hydrogen peroxide completely safe, it is one of those ever interesting and laboratory-equipment-destroying substances. In your case, judging by the casual way you pose the question, I think it would be interesting in a stack-exchange-user-detonating way as well.
Please reconsider whatever you are attempting. The answer is no and no.