demoulation

I have a paper on hand said , "In GMSK Demoulation , this is quite strict and not realizable with a physical Gaussian filter, the phase response can be kept linear and therefore sufficient for coherent demodulation." could u tell me why to be kept linear for phase response? and Could you have papers about GSMK or gaussian filter for my reference, thanks.

You should be able to make a digital synthesis of Gaussian filetr parameters