We have fabricated millimeter-wave GaN high electron mobility transistors (HEMTs) using methyl silsesquioxane (MSQ)-based inter-layer dielectric to suppress current collapse by enhancing the moisture resistance, and removed MSQ around the gate electrode for reducing parasitic capacitance. We clarified that the moisture resistance of conventional benzocyclobutene (BCB) is insufficient to suppress current collapse because water molecules easily permeate BCB, especially when using a cavity-gate structure. On the other hand, in the case of hydrophobic MSQ, the moisture resistance was improved by using a cavity-gate structure, and the current collapse due to moisture was effectively suppressed. So, improving the moisture resistance with hydrophobic low-k films plays a key role in reducing the current collapse of GaN HEMTs with a cavity-gate structure. Moreover, the parasitic capacitance of GaN HEMTs was successfully reduced by using the MSQ-cavity, and RF performance was improved by around 20%.