In this study, the Ga-Te binary system was reassessed by means of the CALPHAD method using a modified lattice stability parameter for Te as well as experimental data for this binary system. The two-sublattice ionic solution model was applied for the liquid phase, and the intermediate phases were described by the sublattice model. A set of self-consistent thermodynamic parameters was optimized for all the phases in the Ga-Te binary system, which reproduced the phase diagram and the thermodynamic properties well. Using the reevaluated Ga-Te system, previously assessed Ga-Se system, and modified Se-Te system, a critical evaluation of the Ga-Se-Te ternary system was performed. The calculated vertical sections, isothermal sections, and liquidus projection agreed reasonably well with the experimental data. Immiscibility in the liquid phase was observed, and the origin of this behavior is discussed from a thermodynamic perspective.