It is simply called "G" Song. It was recorded on 11/11/77. The music was written by Chris Cerf, and the lyrics were by Thad Mumford. It was sung by a lavendar AM Man (Christopher Cerf), and a green AM Woman (Marilyn Sokol), and I'm not sure who the bird was, maybe Jerry Nelson? And the mailman was Richard Hunt.
Here's how it goes:
AM Man:"G"
AM Woman: GGG...
AM Man: Oh, "G"
AM Woman: G-G-G...
Oh, oh, oh
Man:
"G"
How I love that "G"
Listen and you'll see
Why I love that letter
(Starting here, a little bird sings "Oh, 'G'" in the bass line throughout.)
Woman:
Sometimes it sounds like GGG
Sometimes it sounds like G-G-G
Hey, everybody, lets hear it for
My favorite letter
Man:
Oh-oh-oh, oh, "G"
There's nothing else for me
From "G" to shining "G"
Why I love that letter
Woman:
Oh, "G" can be for Grover, Gail, or Glenda-Sue
If George should marry Gina, they'd share a "G" for 2
Nothing could be keener
"G" is thanks to you
That's why I love that letter
Mailman:
I am delivering a "G".
Why are you folks dancing?
Gee whiz!
Now, the mailman starts dancing along with the man and woman, and they all sing their parts at once. It's a wonderful song. And also, this is very interesting. Chris has actually told me that he never actually saw the segment, he just remembers singing on it.