We examined the ZnGeN2-GaN solid-solution system (Zn1-xGe1-xGa2xN2) in the unexplored compositional region of x < 0.10 to reveal the transitional structural and optical properties caused by the introduction of Ga. Fairly stoichiometric fine powder specimens with compositions of x = 0.02 and 0.05 were prepared by the gas-reduction-nitridation method, and their partially ordered Pna21 structure was identified by solid-state 71Ga NMR spectroscopy and time-of-flight neutron powder diffraction. The Rietveld refinement results of the neutron diffraction data showed that the introduction of 2 atom % Ga readily retards the cation ordering in ZnGeN2, and this composition-induced transition to the wurtzite disordered phase proceeds mostly in the range of x < 0.10. The synthesized samples showed gradual red shifts of the absorbance and photoluminescence excitation spectra with their x value, consistent with their degree of disorder, indicating that the narrowing of the band gap achieved in the current system results primarily from the disorder of the cation sublattice accompanied by octet-rule violation, as has been predicted theoretically. The test reactions for photocatalytic water splitting resulted in improved H2 evolution rates of 6.1-72.6 μmol/h under UV-visible-light irradiation, and stable solar H2 evolution of up to 5 days was demonstrated.