BER vs SNR simulation for GFSK modulation (Closed-form BER for GFSK)
Well, GFSK is a whole class of modulations, essentially with your freedom being the choice of symbol duration/pulse shaping filter breadth ($\sigma$ in the Gaussian); of this whole class, M makes most sense, so GMSK is really what you'd most probably would want to deal with.

Now, a BER curve really only makes sense if you can describe the "theoretical" receiver of your signal.

In that sense, you could understand GFSK as "extended" GMSK, where you don't use the shortest possible symbol duration, but a multiple thereof, and hence, would need to adjust the $E_b$ in your $E_b/N_0$ curve. But: there should really just be a factor between the $E_b$ in GMSK and generally GFSK, so if you just shift your curve, you'd get a measure of how well your system compares.