The atomic number of Ga is 31.
The electronic configuration for the neutral atom at ground state is;
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p¹
Once it becomes a cation after losing three electrons, the electronic configuration of Ga³⁺ is;
1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰
The outermost shell in the neutral atom was the 4th energy shell. But after losing the three electrons, the outermost shell is the 3rd energy shell.
the number of electrons in the 3rd shell are - 2 + 6+ 10 = 18
There are 18 electrons in the outermost shell of Ga³⁺.