You can gutload with a mix of oatmeal, bran, ground flax seed, carrots, spirulina, bee pollen and soy protein powder. Some people also use chicken mash (chicken feed) or monkey chow, that sort of thing. Fish flakes can be a good addition too (some of these contain sprirulina, if you can't find it in powder form at your health food store. A health food store where they sell item in bulk (scoop from bins) is a great place to find good gutload ingredients.
Feeding only mealies will also help your geckos from aquiring worms, which are transmitted by crickets. If gutloaded well, they are cleaner and safer than crix and just as nutritious. However I like to feed a variety to keep their appetite and interest up, they will eat more sometimes if you give them different foods. Also, I think geckos enjoy stalking crickets, but that may be a personification on my part. If they don't like crickets, you could try feeding silkworms, butterworms, or king mealworms on occassion for variety. Waxworms are okay as an occassional treat but can be addictive, and are too high in fat to give regularly. Waxworms are "gecko twinkies".