In this paper, novel microwave gas sensors based on graphene-loaded substrate integrated waveguide (SIW) cavity resonators are presented. Two SIW-based cavity resonators, a ring-slot resonator and a complementary split ring resonator (CSRR), are fabricated and coated with chemical vapor deposited (CVD)-grown graphene. The fabricated graphene contains a layer of polymethyl methacrylate (PMMA) on its top. The graphene sheets exhibit high sensitivity to various kinds of polar and non-polar gases. When polar gas contacts the graphene sheet, it will donate or receive electrons, thereby changing its conductance. The SIW cavities thus perform a resonant frequency shift from the perturbation of electron exchange. In the experiment, a frequency shift of 59 MHz and 157 MHz for the SIW ring-slot resonator and CSRR, respectively, can be observed after pure ammonia gas is injected into a closed chamber filled with air at standard atmospheric pressure and temperature. This work demonstrates a very simple and efficient gas sensing scheme in the microwave regime. The proposed devices are promising to be further integrated with RF front-ends, providing a low cost and high sensitive gas sensing and environmental monitoring solution.