Welcome to the forum!
First, this is not the fish we normally call a gulper. What you got is a different driftwood catfish called Iron head catfish aka Trachycorystes trachycorystes. It's a cool fish, rarely kept. Extremely nocturnal, hence your "lazy" comment. Hates light with passion. IMHumO, it is no more sensitive than an average aquarium fish. Neither the whole fish, nor its skin.
The wounds don't look bad right now. I am glad you learned your lesson and you care a lot about your wet pets. I am with others - keep your water nice, stable, and the fish stress-free, and it should recover on its own just fine. No need for any meds. When you do WCs, make sure the water matches, so the fish is not stressed by a sudden change in some parameter, like pH, temp, hardness/TDS, etc. A lot of WCs of high % can cause stress by themselves, so gotta be reasonable. WCs do help with water quality but if the fish is not feeding, I see no need for crazy WCs.
They are not hard to train to take the feed you want them to take but there is no urgency whatsoever. I'd offer what I want it to take, like quality pellets or thawed fish until it takes it. Until then, it can fast no problem. Given the wounds, the fish is stressed anyway and they are stressed, their appetite suffers, so no need to hurry or to worry. If it doesn't eat in a couple weeks, I'd start getting concerned a bit.