The "Lunch Song" is a song from "The Castle". It was also used in a promo for the show.
Lyrics
Gumball: Boom-a!
Gumball: We home alone with Dad so I make me own lunch.
Darwin and Anais: Ooh!
Gumball: I can eat what I want so I take 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 lickle sauce, I say maple and red.
Darwin and Anais: Boom!