It really depends where the gum is... I've never had that happen to me, but you could either ask at your local music store, try... well, you could get a really big freezer and freeze it and chip it off if it's like, on the bell or somewhere easily accessible. Otherwise, I'd go online and look for the biggest, longest instrument snake you can find, get it, and try using that, with maybe some peanut butter on the end? That helps with gum in hair, so I dunno... I'm really a Euphonist, so I don't know much about Tubas... I wouldn't try anything I suggest unless it sounds right to you... the freezing idea actually came from when I got Silly Putty in my hair when I was little... xD