A coordination number can be determined by the usage of an atom towards a molecule from seeing how many numbers of atoms would have to be combined together in an atom.
Gallum: Z = 31
electron configuration: [Ar] 4s^2 3d10 4s2 4p1
Highest energy electron: 4p1
Quantum numbers:
n = 4, because it is the shell number
l = 1, it corresponds to type p orbital
ml = may be -1, or 0, or +1, depending on space orientation, they correspond to px, py, pz
ms = may be -1/2 or +1/2, this is the spin number.