That looks like a pretty young one. I have a gecko, actually the one in my avatar, who developed that. She has a hernia. The vet said that as long as I can pop the lump back in, there are no adhesions and she's OK. For awhile I'd made her a kind of a truss from a bandaid but now she just carts her hernia around. I had to retire her as a breeder but she's doing fine with 2 other retired breeders. She's about 6 years old now. Of course, I don't know that your gecko has a hernia because I just have a side view, but it looks familiar.
+1 for Aliza's answer. I've never seen one in a gecko but it looks a lot like the dozen or so umbilical hernias I've seen on puppies and kittens. Most heal with age - if they persisted surgery was recommended but I'm not sure how effective something like that would be in a bitty gecko. Gently trying to pop it back in and keep it in with light pressure bandage like Aliza says should help. VetWrap sticks to itself and would be a slightly more expensive ($3-$5 for a roll) alternative to a Bandaid. Just be sure to check and change it often (I would say at least twice a week) so it doesn't restrict blood flow as the little guy grows.
I had an adult with that and had to be out down since surgery is extremely risky and it was growing from her liver. Ideas actually a tumor not a hernia in my case.
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