The atomic structure of alpha-7X7 and gamma-phase structure has been investigated with use of scanning tunneling microscopy. Structural models for both phases are presented where both structures are based on the dimer adatom stacking-fault structure. Continuously linked boundaries are observed between these phases, in contrast to those between alpha-7X7 and square-root X square-root 7 phase. It is found that the properties of the boundaries are closely related to the basement structure. Phase transitions appearing at higher or lower temperature are discussed in terms of growth dynamics.