Who is Gabby in the novel, The Year of Billy Miller?

Gabby is the young woman who is Billy and Sal’s babysitter when Mama and Papa go to Chicago overnight for an art show. Gabby is very sweet, very kind, and very caring, She brings Billy and Sal out to eat at Ruby’s Cupboard, and then stays up a little later than usual to play cards with Billy, unaware that he plans to stay up all night.