The specific heats of TGSe and DTGSe crystals, pure and gamma-irradiated, were measured. The influence of radiation defects on the phase transition in ferroelectric crystals was studied by calorimetric experiments. The Landau free energy expansion coefficients were measured for various doses of gamma-irradiation (0 < D < 5 MR). The order of the phase transition changed from the first to the second through the tricritical point in DTGSe crystal.