Assuming you don't know what "gut-loading" is...It is the process of supplementing feeder insects with proper nutrients before feeding to your gecko. I use a mixture of "Flukers Cricket Feed" and "La Pina" corn meal (it has the most calcium in it) with either carrot slices or potatoes for H20. I dust with calcium w/D3 at every feeding, and once a week I dust with a vitamin supplement. And of course like Nick said, you should keep some in a dish in their enclosure. Sometimes they eat it on their own.
As long as it's a formula specifically made for gut loading crickets or other insects, it should be fine. You have to let them eat for a day or two before feeding them to your gecko. Then when they're good and fat, you dust them and feed them to your beast