We present first-principles calculations of the isotropic NMR Ga shielding in metallic MGa2 with M = Ca, Sr, Ba and MGa4 with M = Na, Ca, Sr and Ba. We show that the experimentally observed trend of Ga NMR shifts is as expected driven mainly by the spin part of the response, but the orbital contribution must not be neglected. For all analyzed compounds the spin contact term constitute the major component of the response, except for BaGa2, where the spin-dipolar contribution is unusually large. This spin-dipolar contribution is related to the difference of the Ga-4pz and 4px,y partial density of states (PDOS) at the Fermi level, which is large only for BaGa2. It is related to the honeycomb-like Ga-lattice and the distances between Ga atoms. The spin-contact term is determined to a large extend by Ga-4s PDOS at the Fermi level, because the magnetic field leads to a small spin-splitting and a reoccupation of spin-up and spin-down states. This Ga-4s PDOS is related to the local atomic structure around the Ga atoms and results in fact from an overlap with the neighboring Ga-4p orbitals, therefore more symmetric local arrangements of atoms around Ga result in higher Ga-4s PDOS. However, we noticed that for very low Ga-4s PDOS the spin contact term does not tend to zero but changes sign and becomes diamagnetic. This can be explained by the energy dependence of the Ga-4s radial wave function near the nucleus, leading to a contraction/expansion of 4s densities, respectively. This effect is also present in all insulating materials; however, it has been neglected so far in literature.