Understanding GFSK signal
When you consider the signal Bandwidth and slew rate of the VCO, how much of the time it spends at 100% input voltage and the deviation ratio of FM/data rate shapes the spectrum in between and thus the the end points may be reached in a small portion of the time. 

Examine the amplitude with zero f sweep say 20 MHz off centre to get an AM response of the FM with no video filter and 10MHz resolution and see what your data looks like on XX us / div. This is a poor man’s FM discriminator on the slope of the filter.  then examine phase shift, ISI and group delay when centred.