Surface phase diagram of recently proposed GaAs(0 0 1)-(2 x 4)gamma is systematically investigated by using our ab initio-based approach. We focus on the (4 x 7) domain consisting of c(4 x 4)-like and (2 x 4)-like regions to clarify surface dimer constituents as functions of temperature T and As (As-2 and As-4) pressure p(As) by comparing chemical potentials of surface dimers in the vapor phase with that on the surface. The calculated results under As-4 imply that Ga dimers in the c(4 x 4)-like region tend to become stable with increase of temperature and appear at the conventional growth condition such as T similar to 800 K and p(As) similar to 10(-6) Torr, while the (2 x 4)-like region favors As dimers. This is consistent with temperature dependence of change in surface dimer constituents on the c(4 x 4) and (2 x 4)beta 2 clarified in our previous study. Furthermore, the surface phase transition from the c(4 x 4) to (2 x 4)beta 2 via (2 x 4)gamma is discussed on the basis of the phase diagram obtained in this study. (C) 2008 Elsevier B.V. All rights reserved.