The newly synthesized novel double-Ga layer nanolaminated carbide Mo2Ga2C is stimulating tremendous research interests. In the present exploration of Mo2Ga2C structure, another plausible metastable structure with close-packed Ga layers is predicted from density functional calculations. This new structure (denoted as m-structure) is slightly less stable by only 52 meV/atom than the experimentally determined structure (e-structure) with on-top stacked Ga layers. Importantly, this m-structure is dynamically stable from the calculated phonon dispersion. Moreover, the stability behavior of this m-structure under compression shows that possible phase transition from e-phase to m-phase could occur under a pressure above 24.3 GPa, which requires further experimental confirmation. Phase transition model is proposed, and the energy barrier for phase transition is further derived. The electronic structures show that Mo-C bonds and Ga-Ga bonds are weaker in metastable m-phase than in e-phase. During compression, more strengthening of Ga-Ga and Mo-Ga bonds and less weakening of Mo-Mo bonds in m-phase explain the stabilization of m-phase under pressure.