A simplified modal method to explain the resonance phenomenon in guided mode resonance (GMR) gratings with asymmetric coatings is presented. The resonance observed is due to the interaction of two propagation modes inside the grating. The reflectivity spectra and electric field distributions calculated from the simplified modal method are compared using rigorous coupled-wave analysis (RCWA). The influences of high-order evanescent modes on the resonance peak are analyzed. A matrix Fabry-Perot (FP) resonance condition is developed to evaluate the resonance wavelength. An explanation for the resonance phenomenon observed based on the FP resonance phase condition is also proposed and demonstrated. The simplified method provides clear physical insights into GMR gratings that are useful for the analysis of a variety of other resonance gratings.