I'm glad to hear that you have found a fish you have been looking for. Obviously, a 50 g tank for any 11" fish is hardly ideal, much less an open-water predator that has a known aquaria history of needing LARGE tanks. But my preaching will not help any now.
So, don't reinvent the wheel if you don't need to. Why not tape dark paper to the sides of the 50g, so the GATF can "see" there is something there, and then just leave him to treat with salt and water changes. Move the bottom dweller, not the GATF; trust me, it will be much easier.
I hope you got the GATF cheap, otherwise you should have gotten a large temporary tank/vat to hold him in the interim. Be certain to keep up the water changes and check your parameters daily. If they are good, you'll be astounded how quickly damage heals on these fish!
EDIT: After seeing that pic, that is very bad. I would recommend a contact antibiotic (NOT AN ANTIBIOTIC BATH!!!), but take great care in using it on that type of damage. I hope this turns out well for you, and perhaps the GATF's upper mouth will regrow. Now I would go sternly reprimand whoever told you keeping that fish in a 50g was a good idea.