Gyrosynchrotron emission generated by non-thermal
electrons in solar and stellar coronal flares can be efficiently modulated by slow magnetoacoustic oscillations in the flaring loops. The modulation mechanism is based upon the perturbation of the efficiency of Razin suppression of optically thin gyrosynchrotron emission. The modulation of the emission is in anti-phase with the density perturbation in the slow wave. The observed emission modulation depth can be up to an order of magnitude higher than the slow wave amplitude. The modulation is more pronounced at lower frequencies. Observations with spatial resolution together with the analysis of the modulation frequency are shown to be sufficient to provide the information sufficient for the mode identification.