I too would think a growth. How long has it been there? Has it grown? Does the fish seem to be in good health otherwise? It might also be useful to know: the size of tank, the number of fish, the water parameters (if you test the water), your maintenace routine and how long the tank has been set up.
If it is a growth then there is little to be done. Surgery is an option but vets that treat fish are few and far between. Experienced fish-keepers can attempt surgery on there own but since it is located on the gill cover I would not recommend it. If it doesn't seem to be harming him then it can just be left alone (providing there are no enviromental issue that could be the cause).

I checked through the many books I have but there is little information, one picture did match though. The tank size is good, your maintenace routine is not ideal but since he has been healthy these last five years it is most likely not the cause. I would recommend taking a water sample to a pet store to have it tested (this is usually free), just to make sure that everything is ok. I'm sorry I couldn't be more help.