I wouldn't put either of those fish in there for life. I would turn that into a small community tank. Maybe some RAM cichlids or something.
Maybe not the gulper, but I have seen time and again people keeping Erythrinus in both 20g L, and 29g's, they seem to fare pretty well. Although as with any fish, the bigger the tank the better.
My wolf, http://www.monsterfishkeepers.com/forums/showthread.php?437546-Erythrinus-Sp.-(Peru)-quot-Purple-Wolffish-quot