Gallium has 31 electrons (₃₁Ga 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹) and if loses 13 electrons, gallium would have configuration of noble gas argon with 18 electrons (₁₈Ar 1s² 2s² 2p⁶ 3s² 3p⁶), but that is not possible, so gallium only loses three electrons and has electron configuration:
₃₁Ga³⁺ 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰.