Using equivalent transformation and spin-rescaling methods, the Gauss model on a decorated square lattice is studied. It is found that the square decorated Gauss lattice could be transformed into a regular square Gauss lattice with nearest-neighbor, and next-nearest-neighbor interactions. By calculating the regular square-lattice Gauss model, the critical temperature of the Gauss model is obtained on a decorated square lattice, and the exact phase diagram of this system can also be obtained.