The real and imaginary bandstructures of deeply scaled GaN nanowire (NW) field-effect transistors (FETs) are calculated with an eight-band k∙p model. Analysis of the transport properties of both GaN NW FETs and NW band-to-band tunneling FETs (TFETs) is presented. Deeply scaled n-type GaN NW FETs operate in the classical capacitance limit (CCL) in stead of operating in the quantum capacitance limit. This is a result of the high electron effective mass and high density of states. We discuss how the CCL operation of these devices affect the device performance. For the GaN NW FETs, within a source Fermi level of 0.2 eV, the current density varies from 5 to 8.5 A/mm. For the GaN TFETs, we present analysis on the effect of NW diameter on the on-currents, the off-currents, and the required electric fields. We show that a drive current of ∼0.05 A/mm can be achieved for the GaN NW TFETs. These devices show potential for high-speed and high-power applications.