Welcome aboard
How long have you had the Gulper Catfish? If you just got the Catfish recently allow it to settle in. Eventually it will eat when it's ready.
Agreed. With these cats they can go upto 2-3 months without eating and be fine. It's the main reason why they have such an expansive stomach. These fish in the wild will sit in one spot and wait for days -weeks-months for prey to swim right infront of its face. Even if the prey is longer then the cat itself is, it will still eat it.
I once seen a video about this guy feeding his 10in gulper cat 3 8in koi all in one sitting. That cat ate every single one of those koi and looked like it was about to burst. The guy also said that he feeds his cat like that once a month, but other then that large feeding it doesn't eat for the rest of the whole month. Now don't take it the wrong way, I'm not suggesting that ya do what he did, I'm just saying this as a show of how long these cats can go without food.