Gold gratings were measured by spectroscopic ellipsometry and modeled by the finite element method to investigate the capabilities of optical dimensional metrology for plasmonic diffractive structures. The gratings were prepared by electron beam lithography using parameters determined by finite element simulations for significant variations of the amplitude ratio and phase shift of the polarized reflection coefficients to achieve high sensitivity for both the measurement of the grating dimensions and the sensing capabilities. Sub-nanometer sensitivity was shown to determine the grating dimensions and the thickness of an adsorbed layer to be detected in both traditional reflection and Kretschmann-Raether (KR) configurations. The sensitivity for the refractive index of the ambient was calculated to be 10−5 at best, which is not significantly better than the sensitivities for plane gold layers in KR configurations. However, in diffraction-based resonant setups, the high sensitivity dips can be shifted to a larger spectral range, which is highly significant in many applications. It was also revealed that 2D models assuming a perfect geometry fit the measured ellipsometry spectra only qualitatively, leaving room for model development in the future.