Yes, you can put a 30gal heater in a 6 gal tub. In a pinch, I've used 50w heaters in 2 gal tanks. Can you get a pic of Gilbert? I might be able to tell if it's a tumor or infection.
For treatment, 3 tsps of epsom salt per gallon and Indian Almond Leaves or oak leaves if you have them. How is he acting otherwise? Is he eating and acting normally?
On a related note, my mom's fish Stumpy (she claimed him as hers) is pineconing. Still can't tell if it's camallanus worms or something else. His red ventrals get in the way.