Compounds forming between silicon gallium and nitrogen
The ASM Alloy Phase Diagram Database (ASM Database if your institution has access) shows GaN and Si$_{3}$N$_{4}$ as the only compounds in that system. Standard binary alloy references agree. No ternary compounds are listed. Given the difference in crystal structures between all end members and the two nitride phases, mutual solubility of any two phases seems unlikely (little to none exists in the binaries, even Ga in Si has a pretty low solubility).