The official Century 21 Brookings app is here! Gustafson, Krogman, and Associates Realty, Inc of Brookings, SD wants to help real estate sellers and buyers. The Century 21 Brookings, SD app contains listings of homes for sale, easy contact, a list of open houses, agent bios, and much more. Anyone considering the purchase or sale of a home in Brookings, South Dakota should consider downloading the GKA Realty Century 21 mobile app.