In this paper, we compared the results of a heat capacity study of an erbium-doped gallium gadolinium garnet crystal with data for an undoped garnet. The measurements were carried out in the temperature range from 1.9 to 220 K and in magnetic fields from 0 to 9 T. The temperature dependences of the specific heat were interpreted with allowance for the Schottky contributions due to the Gd ³⁺ and Er ³⁺ ions and the contributions of the thermal vibrations of the crystal lattice. The values of entropy and magnetic entropy are calculated.