The little guy is a fighter
If it is a parasite, the external symptoms are likely secondary. When an animal is weakened it can be attacked by all kinds of pathogens, for example, I've seen true fungi secondary infections emerging and some other "interesting" things.
Now, a couple of tidbits of my experiences with parasites (worms): Gastros do appear to be more vulnerable than other hillstreams...probably has to do with their eating habits and smaller size. It does not mean that other species are safe, it just takes longer. F.e. I saw a pattern of six Gastro deaths in one tank and no deaths among other species.
The very best thing you can do is to try to locate someone who can figure out what you are dealing with exactly. If the animal dies, necropsy often provides the answers. If the animal pulls through, poop can be examined. In my last encounter, I was unable to stop the dying until a professional looked at a dead Gastro and identified the problem. See if there is a vet that deals with fish around.
What is the sequence of dying exactly? If you had a few deaths at the beginning, and then 3 months of quiet, a worm is less likely. If you had continuing deaths, say every one/two weeks, a worm is quite possible.
Now, if the slime coat problem is the primary infection (imho unlikely, but who knows?), and if it is bacterial, furan2 is the best med to deal with such problems. Since it is pretty much biofilter-safe and non-toxic to fish, it is something that can be tried. If this med is not available, anything based on furandazole can be tried, e.g. Furanese.
hth & good luck!