NPR's guide to the running gags from the show.
G.O.B. loses a tooth, turning sibilance into whistling.
2 episodes feature this joke:
G.O.B. gives Michael the animation rights to Mr. Bananagrabber, a cartoon character who whistles like G.O.B. with a missing tooth.
Related Joke: Mr. Bananagrabber
Sources: Joke data compiled by Adam Cole / NPR. Episode metadata from The TVDB via a Creative Commons Attribution 3.0 United States (CC BY 3.0 US) license.