We present a novel approach for growth of surface-directed spinel ZnGa₂O₄ and β-Ga₂O₃ nanofins coated with a non-polar GaN shell. Our results show that the use of a binary compound such as core–shell nanostructures as a starting material is not necessary to promote the Kirkendall effect. Our starting materials include ZnO fins and gaseous Ga atoms under nitrogen plasma. The surface-directed ZnO fins provide a spatial confinement to the Kirkendall effect by which the Zn is exchanged with Ga via a vacancy-assisted mechanism. The results show that at a higher Ga concentration, the wurtzite ZnO fin is converted to a ZnGa₂O₄ cubic spinel, while a lower Ga concentration leads to monoclinic Ga₂O₃ fins. The fin transformation is followed by GaN shell overgrowth on the fin side walls with a non-polar surface. Within the newly formed fins, we observe uniform Kirkendall nanochannels that are laterally formed between their side walls. This method offers the opportunity of growing heterojunctions of a broad range of wide bandgap spinel materials with GaN. The predictability over surface registries of the core/shell fins and their tunable porosity are anticipated to be of significance in a wide range of applications in chemical- and electro-optical based sensing as well as high power electronics.