We demonstrate high linearity of fully processed GaN on 2-mm high resistivity (HR) Si wafers with high effective resistivity (\textbackslash{}rho\{\textbackslash{}mathbf \{eff\}\} > 2 \textbackslash{}text\{k\}\textbackslash{}Omega \textasciitilde{}\textbackslash{}cdot cm). Using two common-gate (MOS-)HEMT devices presenting different levels of distortion as demonstrators, we discuss the impact of substrate-induced harmonic distortion (HD) on RF switches. By comparing switch and CPW line measurements, it is possible to estimate the substrate contribution to HD. For the relatively nonlinear device, the intrinsic device nonlinearities dominate by a 16-20-dB margin over the substrate contribution. For a more linear device (-110 dBc of second harmonic power at 900 MHz and \{P\}\{\textbackslash{}mathbf \{in\}\} = 15 dBm), the substrate distortion contributes equally to channel nonlinearity, highlighting the necessity of engineering the substrate's RF performance together with the transistor. We provide guidelines for substrate \textbackslash{}rho\{\textbackslash{}mathbf \{eff\}\} to achieve good switch linearity for given device harmonic levels.