Magnetoplasmonic gratings offer a suitable platform to analyze fundamental aspects of the interaction of plasmons with magnetization. In particular, broken time-reversal symmetry induces frequency shifts in plasmon resonances that result in large magneto-optic responses at the plasmon frequency. Here we study the excitation of plasmons with light at oblique incidence, focusing on the effect of plasmon excitation on the diffracted modes of the gratings. This way, we were able to select one of the two counterpropagating plasmons allowed for each diffraction order. Interestingly, in these conditions we found large magneto-optic amplitudes driven by plasmons near grating diffraction modes. Finite-difference timedomain simulations confirm these experimental results and corroborate that the sign of the magneto-optic enhancement is exclusively dependent on the surface plasmon wavevector and does not depends on the angle of incidence.