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Electronic structure and exchange interaction have been calculated for the Gd4Bi3 magnetocaloric compound and Gd metal by a local-spin-density- approximation + Hubbard U (LSDA+U) method. In Gd4Bi3 and Gd, the 4f states have little direct contribution to the ferromagnetic coupling. The higher Curie temperature is ascribed to the strong interaction between the Gd 5d and the Bi 6p electrons in the Gd4Bi3 compound. The estimated Curie temperatures from the calculated effective exchange coupling constants based on mean-field approximation are 337 K and 403 K for Gd metal and Gd4Bi3, respectively. These calculated values are comparable with the experimental values of Gd (T C = 294 K) and Gd4Bi3 compound (T C = 332 K).