According to Greg, the song is Brett's version of "Take The Money And Run" (1976) by the Steve Miller Band.
Billy Gnosis is a tribute to Billy Pilgrim, the protagonist of Kurt Vonnegut's novel "Slaughterhouse-Five" (1969). The song Billy, on the other hand, is more autobiographical.
The line "and the worms ate into his brain" is taken from the song "Hey You" (1979) by Pink Floyd:
But it was only fantasy
The wall was too high as you can see
No matter how he tried he could not break free
And the worms ate into his brain
Parts of the song have been played live a few times. In 2010, the whole song was played as part of an 80's set.
^ The Big Takeover #55 (2004)