The effects of access region scaling on the RF performance of millimeter-wave GaN HEMTs is investigated through full band Cellular Monte Carlo simulation. The nanoscale carrier dynamics under the gate controlled region is simulated in devices with different access region lengths in relation with their cutoff frequency. In particular, the cut-off frequency is shown to increase monotonically up to 860 GHz by symmetrically scaling the source to gate and gate to drain distance from 635 nm to 50 nm. The electron scattering rates have been studied showing that while polar phonon emission is the overall dominant scattering mechanism, the emission of acoustic phonons is greatly enhanced in shorter devices. By scaling the gate length and the access region at the same time, we find that the cut-off frequency increases further. Moreover, for vanishing access regions, we are able to calculate a maximum velocity, while a limit effective gate length has been obtained for the physical gate length approaching zero. Based on these limits, we calculate the transit time and find a limiting cut-off frequency of 1.49 THz for the GaN HEMT studied in this work.