Herein presented is a numerical treatment of plane-wave scattering by gratings with metallic strip linings over both exterior surfaces of each conducting bar to create flanged groove apertures, which altogether is covered on both sides by multiple dielectric layers. The technique hinges on the method of moments that employs parallel-plate waveguide cavity dyadic Green's functions and a numerical spectral Green's function for planar stratified media along with Floquet concepts as well as the PEC equivalence theorem. In terms of reflection and transmission characteristics as well as surface wave modal dispersion, results computed by codes written based on this formulation are validated with those simulated by a commercial solver as well as in literature. Arising from the special complex geometry, the additional degrees of freedom offer measures for enhancing the performances of various applications such as beam deflectors, resolution of spectroscopic gratings, grating couplers, and grating pulse compression/decompression.