I'm assuming Vin is the output of the error amp and delta Ve is the peak to peak range of the oscillator ramp, but without seeing a schematic it's hard to be sure. Also not sure what you mean by the filter.
That equation by itself doesn't make sense then. It should take a form like Vout=Vin*D*Ns/Np, with D expressed as Verror/Vramp or something. In that equation the gain is just Vout/Vin=G=D*Ns*Np. But that equation takes neither of those forms, and I don't see how it could be correct.