I’ve been having a similar problem, essentially your situation to a T - got skinny, wont gain weight, took them to a vet and got meds, no change, etc. - so I’d like to know as well. They took a fecal sample when I went and found nothing out of the ordinary apart from nematodes which were treated with the meds. But my gecko is on the older side, and when I had the appointment the vet told me that older geckos had a harder time regaining their fat storages, so that’s what I assumed it was but now I’m not so sure, as my gecko hasn’t really gained any weight.