Determining the highest temperature attained by a magnetic material in the adiabatic magnetization is important for the optimal selecting of magnetocaloric material. As an example, the Gd3Ga5O12 crystals are investigated. Under the superstrong magnetic field and at low temperature, the form of effective magnetic susceptibility is given based on the tendency-saturation law. The magnetic entropy change and the phonon entropy change as well as the magnetocaloric effect are calculated in a magnetic field range from 0 to 40 T. The calculated results are in good agreement with the measured data. A method of determining the highest temperature attained by magnetic material in the adiabatic magnetization is given by using the only intersection point between the curves of the saturation-magnetic entropy change and the phonon entropy change. The highest temperature in the adiabatic magnetization is predicted to be 64.7 K for the Gd3Ga5O12 crstal.