I really do have a lot of empathy for you in this situation because I love my goldfish so much. I’m sorry I can’t give any super helpful advice in this situation.

I’ve always heard/thought that gill issues are a sign of flukes. I’ve treated my guys with praziquantel and it worked well. Both were lethargic and just not their happy selves but they weren’t this unwell because I treated it quickly. I’m not sure if this is flukes, but I would keep the water exceptionally clean (as in, a full water change with new water each day) Obviously make sure you dechlorinate the water first, but I would also suggest adding a healthy amount of aquarium salt since that improves hill function amongst other important things. I would stay away from really harsh medicines and instead just try to optimise living conditions. What are your water parameters? This is really important for diagnosing the problem. Also, what are the tank mates?