Gfsk demodulation of signals with non constant amplitude

I have a real GFSK signal that occupies 500KHz whose center frequency could be, say 20 MHz. Not doing the diff(angle(Y)) since arcTan is CPU expensive and I am hoping to get solutions to port to real time programs. But one file after passband filering doesn't have a constant amplitude. The algorithms I find require a constant amplitude. There is a power up in the signal. How do I transform this GFSK signal to have constant amplitude (at least locally in sections)? Fs=100e6.Would the answer differ much if the signal were IQ Baseband?