One possible bacteria that can make ulcers like this is Aeremonas. A nasty infection. Quarantine tank for sure.
You can treat it externally with either of these methods, but ONLY ONCE.
Catch the fish, and hold him so the wound is exposed to the air and put on a single drop of hydrogen peroxide OR one of the vaseline based antibiotics like Neosporin. Use a cloth soaked in stress coat to hold him.
If he is still eating then use antibiotic treated food, perhaps an antibiotic like Kanamycin.
If he is not eating then use an antibiotic that will be absorbed from the water.
I would also add a small amount of salt to the water, 1 teaspoon per 10 gallons to help with stress.
Keep the Q-tank bare bottom and vacuum daily to remove fish waste and uneaten food. Bacteria can live in debris.
Keep the tank warm, and the air above it warm. Gouramis are labyrinth fish, and need warm air above the tank.
Fish are in less stress in darker surroundings, and if they have a place to hide. I would trim the plants in the main tanks and let the trimmings drift in the Q-tank. They will make some shade and hiding places near the surface.