The aim of this work is to compare the structural and electronic properties of GaH3 Lewis acid to its BH3 and AlH3 homologous hydride of the 13th group in the presence of a donor electron-pair as XH3 (X=N, P, and As). The structures and coordination mode of H3GaXH3 complexes are studied. Compared to its homologous hydride, the isolated and complexed GaH3 is characterized by the implication of its metallic property via d atomic orbitals in both geometrical and coordinating behaviors. Ga–H bond length does not correspond with the intuitive predicted increase in B–H, Al–H and Ga–H series. This is due to the increase of σ character of this bond. 3MO-4e and 2MO-4e models accounts for H3B–L and H3Al–L (L=donor ligand) complexations but the H3Ga–L coordination is governed only by the first one which implies one of d atomic orbitals of the gallium. These phenomena have been shown by the analysis of molecular orbitals of the donor and acceptor fragments. All calculations have been performed at B3LYP/6-311+G(3df,2p) level of theory.