Adsorption-desorption behavior of Ga on GaAs surfaces such as (001) and (111)B is systematically investigated using our ab initio-based approach, which incorporates chemical potentials in the gas phase of Ga atom or As molecules as a function of temperature and their beam equivalent pressure. Surface phase diagram calculations of the (001)-c(4×4) reveal that the c(4×4)α consisting of Ga-As dimers independently appears on As₂ and As₄ in the gas phase, whereas the c(4×4)β with As dimers is destabilized under As₄. On the basis of this finding, surface phase transition between c(4×4) and (2×4)β2 is discussed in terms of 0.5 monolayer of Ga predeposition. Furthermore, surface phase diagram calculations of the (111)B-(2×2) imply that Ga adsorption induces As trimer desorption on the (2×2) surface. This suggests that Ga adatom acts as a self-surfactant atom to maintain layer-by-layer growth of GaAs without anti-site defect formation.