Gwen's official American Girl wiki states:
Gwen and her mother Janine fell on hard times when her father lost his job; they later lost the house as they were unable to keep up payments. Soon after, Gwen's father left them and they became homeless the fall before the start of the book's events. Initially, Gwen's mother has them live in their car until the winter comes; she then takes them to Sunrise House, a place for homeless women and children. Sunrise House helps them get on their feet and eventually get a new apartment.
http://www.youtube.com/watch?v=MY320_pnxVs