Utilizing first-principles band structure method, we studied the trends of electronic structures and band offsets of the common-anion heterojunctions GaX/ZnGeX₂ (X = N, P, As, Sb). Here, ZnGeX₂ can be derived by atomic transmutation of two Ga atoms in GaX into one Zn atom and one Ge atom. The calculated results show that the valence band maximums (VBMs) of GaX are always lower in energy than that of ZnGeX₂, and the band offset decreases when the anion atomic number increases. The conduction band minimums (CBMs) of ZnGeX₂ are lower than that of GaX for X = P, As, and Sb, as expected. However, surprisingly, for ZnGeN₂, its CBM is higher than GaN. We found that the coupling between anion p and cation d states plays a decisive role in determining the position of the valence band maximum, and the increased electronegativity of Ge relative to Ga explains the lower CBMs of ZnGeX₂ for X = P, As, and Sb. Meanwhile, due to the high ionicity, the strong coulomb interaction is the origin of the anomalous behavior for nitrides.