I used to have a gourami with the same problem. He managed to live with this tumor for a year or so. In summer, when we were all away from home, he disappeared.
Then, in about 6-8 months one of my blue neons got something similar but it was not near a gill. I tried to treat the poor guy (I put him into a quarantine tank) but it was too much for him, although I followed the instruction closely.
I have found out that if the tumor is close to the gill or anywhere near the neck it could be due to the lack of iodine, just like in people. Then it could probably be treated, but professional advice is needed. I will try to look for the site which does that.