GaN/V2O5 core-shell nanowire (NW) piezoelectric generators of controlled crystallographic orientations were fabricated, and the device performance was characterized. Catalyst-assisted c- and m-axis GaN NWs were grown on a c-plane GaN thin film by varying the NH3 flow rate and reactor pressure. The grown NWs were then utilized to fabricate the flexible piezoelectric nanogenerators (PNGs) to practically investigate the impact of the c- and m-axis GaN NWs on the piezoelectric response. The c-axis GaN NWs exhibited a higher piezoelectric output than m-axis GaN NWs. Furthermore, the GaN/V2O5 core-shell structure was utilized in the NWs to suppress the internal carrier screening that degrades the piezoelectric output. The maximum output voltage (27 V) exhibited by core-shell c-axis NWs was thrice the voltage exhibited by pristine c-axis GaN NWs (9 V). A stability test was performed for one hour to verify the feasibility of using flexible PNGs for real applications. The high stability of PNGs was attributed to the flexibility and high crystallinity of the NWs.