in that before the baseband waveform (with levels -1 and +1) goes into the FSK modulator, it passed through a Gaussian filter to make the transitions smoother to limit spectral width. Gaussian filtering is a standard way to reduce spectral width; it is called pulse shaping in this application.

In ordinary non-filtered FSK, at a jump from -1 to +1 or +1 to -1, the modulated waveform changes rapidly, which introduces large out-of-band