Why is the GMSK modulation constellation round?

The phase change of GMSK signal changes continuously on a circle.

Is GMSK FM keying modulation? Type. That is to say, the frequency of the modulation signal varies with the modulation signal? Has FM changed? Not phase modulation.

GMSK modulation method is to add a baseband signal preprocessing filter before MSK modulator, that is, Gaussian low-pass filter. Because this filter can convert baseband signal into Gaussian pulse signal, its envelope has no sharp edges and inflection points, thus improving the spectral characteristics of MSK signal.