LiGayCo1-yO2 solid solutions, obtained under a high-pressure (3 GPa), were studied by high-frequency electron paramagnetic resonance (EPR) of Ni3+ spin probes and Ga-71 MAS NMR. Ga-71 MAS NMR experiments exhibit complex spectra that correspond to octahedral coordination and quadrupolar effects. For Ga-substituted oxides, high-frequency EPR of Ni3+ spin probes shows the existence of two new kinds of Ni3+ ions in tetragonally elongated octahedra with g(perpendicular to) > g(parallel to). These ions are characterized by slightly different g-tensor and temperature of transition from dynamic to static Jahn-Teller effect. The extent of the local tetragonal distortion has been deduced from g(parallel to). The reinforcement of the tetragonal distortion of NiO6 for LiGayCo1-yO2 as compared to LiCoO2 reflects the different structure characteristics of the GayCo1-yO2 and CoO2 layers. For Ga-rich composition, X-band EPR spectra exhibit a new signal with g approximate to 5 ascribable to high-spin Ni3+ located near defect sites. As compared to the Ga-free LiCoO2, a clipping effect in the slabs induced by the Ga inhibits the expansion of the c lattice parameter when LiGa0.1Co0.9O2 is oxidized.