Feeder isopods are common for dart frogs, as dart frogs are about the size of a quarter and the feeder species of isopods(dwarf white, purple Costa Rican) are the size of ants. The smaller species of isopods breed at a reasonable rate (a month or two till the new borns are feeder sized) , where as the larger isopods (giant canyon) that can grow up to almost an inch in length take a few years to get up to a size. None of my geckos give the isopods in their vivariums a second look, as the isopods are too small to consider food. I use, white dwarf, Costa Rican purple and bright orange sow bugs (as a clean up crew, to eat gecko waste and the leaves that fall off my plants). The orange sow bugs are the largest I have, at a half an inch in length at full grown. All my geckos are much smaller than a even a juvenile leopard gecko.
So to answer your question, "can you use them as feeder?" , well you can try, I'm sure if you feed your gecko out of a glass dish that the isopods cannot climb out of, your gecko might be curious what these tiny specs are and try to lick some up. But if you release the isopods into the tank, they will immediately hide and then suffocate (they have gills and breath water vapor in the air, not the oxygen, there for anything less than let's say 50% humidity is like drowning them in air. sounds weird I know, but it's true)
"Are they nutritious?", I wouldn't worry about that unless your talking about using them as a staple feeder every day. Variety and everything in moderation is the key to good health. But I would say they are similar to tiny roaches (at least by physical appearance) although since they have gills, maybe comparing them to crickets or roaches, is like comparing the nutritional value of fish meat to chicken meat. Again variety is key, the more options your gecko has, the happier and healthier it will be.
I bought all my isopods off
buy live feeder insect isopod springtail and roach they have different bugs on there periodically so, check from week to week to see more feeder(or pet bug) options.
