I've seen this happen before in a group of about 70-100 young guppies that I had bred (no direct relation to imports that I know of). Their eyes and body seem to darken slightly- as you can see in the last photo of the one resting on the bottom, they get the shimmies, clamped fins and flick themselves periodically, they rest on the bottom of the tank or on a leaf and they don't really gasp as such but they sort of have their mouth hanging open a bit also like in the last photo of the guppy resting at the bottom. But strangely they still ate food as they did normally. I have no idea how this "mystery" disease was introduced into my tank as no other fish had been introduced or plants/ornaments. Ammonia, pH etc was normal. I tried treating with salt, meth blue Wonder Tonic stuff, increasing temp, water changes, changed some of filter media etc nothing worked, every single one died over about two weeks :tears: (there was no other species of fish in the tank).
So good luck and I hope you find a cure to this, keep us posted as I'm interested to find out if it affects other fish and if there indeed a cure if it ever happens to me again.