The "Lunch Song" is a song from "The Castle." It was also used in a promo for the show.
Lyrics
Gumball: Boom-a! We home alone with Dad so I make me own lunch.
Darwin and Anais: Ooh!
Gumball: I can eat what I want so I pick the best munch.
Darwin and Anais: Ooh!
Gumball: I pick up ham and chocolate chips and stick 'em in the bread.
Darwin and Anais: Ooh!
Gumball: It need a little sauce, I say maple and red.
Darwin and Anais: Boom!