If it is a prolapsed rectum there's very little you can do, other than fast the fish for several days to see if there ia any improvement if not you could then try an Epsom salt bath to act as a laxative/purgative to try and elminate any obstruction. Hopfully the condition will reverse itself. If possibile it would be best to do this in quarinteen so you can monitor it's intake and keep the water very clean to prevent the sore from becoming infected or having it picked on by another fish. After the fasting, if it sems to be looking better feed a couple skinned green peas to help make sure the disgestive track is moving things along. If the current diet doesn't containe vegitable matter make sure to incorperate it into it's future diet, GTs seem to benifit from some veggies added to their diet. In some instances it will reverse itself sadly most do not. I had this happen to an angel fish and it didn't make it, I hope you have better luck. That's about all the advice I have maybe someone else has some additionl advice. Good luck.