The Ce(Ni1-xGax)5 system has been investigated in the range from x=0 to x0.6 including measurements of (a) the lattice parameters at room temperature and down to 4 K on samples quenched from 600 or 800°C, respectively, (b) the magnetic susceptibility in the temperature range 1.5 <T<800 K and magnetization measurements up to 8 T, (c) the electrical resistivity from 4.2 to 300 K, (d) the specific heat from 1.5 to 60 K and (e) x-ray absorption spectroscopy (XAS) and x-ray absorption near-edge structure (XANES) at 10 and 290 K. At 800°C, a continuous solid solution 0x0.55 is observed crystallizing with the CaCu5 type. Within this solution range a HoNi2.6Ga2.4-type low-temperature modification develops with a small homogeneity range around CeNi3Ga2 and with a maximum congruent transformation point at T770°C. The Ni/Ga substitution leads to a successive occupation of the 3g sites of P6/mmm symmetry by Ga, whereas the 2c sites are Ga free, but ultimately does not reach full atomic order at a hypothetical CeNi2Ga3. Magnetism within the solid solution starts with a nonmagnetic ground state of the cerium atoms in CeNi5 but subsequently changes to an intermediate Ce valence as the Ga content increases. CeNi3Ga2 in the low-temperature modification exhibits Curie-Weiss behavior and enhanced values for the electronic contribution to the specific heat as typically observed for heavy-fermion compounds. The Ce valency, directly obtained from XAS data, shows a linear decrease as a function of increasing Ni/Ga substitution, with a strong temperature dependence for the alloy richest in Ga, CeNi2.34Ga2.66. The linear variation of the unit-cell parameters for (Y,La)-substituted alloys Ce1-x(Y,La)xNi3Ga2 in both structure modifications and the corresponding magnetic and XAS data revealed practically no influence on the cerium valence compared to the CeNi3Ga2 base alloys.