"Gonna Fly" is a parody of Enya's popular song "Orinoco Flow (Sail Away)". It was featured in the Season One episode, "Death".
Background
Marvin Marsh just turned 102 years old and is miserable. He tries to persuade his grandson, Stan Marsh, to murder him through assisted suicide. In an attempt to show Stan what it is like to be old, Marvin plays to him a cassette tape of this song in a dark, locked room.
Stan describes the song as "Cheesy, but lame and eerily soothing at the same time." Which is what Marvin was trying to convey regarding his own life as a century old man.
Lyrics
I'm Gonna fly, I'm gonna fly. I'm gonna fly.