We investigate the effect of varying the gate-to-drain spacing and the gate field-plate on the device linearity of GaN HEMTs on Si for 0.11μm, 0.15μm, and 0.19μm gate lengths. The gain compression, phase distortion, and harmonic distortion metrics are measured using a nonlinear characterisation setup calibrated at 6GHz up to the third harmonic. The acquired nonlinearity metrics are correlated with the extrinsic device parasitics extracted from S-parameter measurements. We observe that excessive gate field-plate length scaling down to 0.05μm lowers the total phase distortion at the expense of gain linearity and harmonic distortion in Class AB while minimising the gate-to-drain spacing alleviates the harmonic distortion only for devices of 0.19μm gate length. Further evaluation, under matched conditions, using a passive load-pull measurement setup points to a decline in the peak achievable PAE and PSAT at gate field-plates smaller than 0.12μm.