This is the first time I hear of gulper eating a gulper. Do you believe the poster or not?
Cannibalism is certainly not unheard of in auchenipterids, and I wouldn't be surprised if it did indeed happen; I do not recall any particular instance of gulpers of vastly different sizes ever being successfully cohabitated, though I could be wrong. Neither gulper in the video seems to be in the best of health (the smaller one seems quite ill), which may have some impact.
*Edit: Predatory Fins has successfully cohabitated gulpers of multiple different sizes in the same tank with success, or so it seems from some of their past videos. I don't doubt that the gulper in the video posted ate the other, but I do not believe it was solely due to predatory instincts; something else likely had some impact. Then again, I'm likely incorrect on the matter.
Hardly the time or place for this, but I utterly detest the owner of that poor fish- from what I can see, they're merely stressing the animal out by housing it with unreasonable tankmates, and deliberately anthropomorphising for the sake of views.