Confusion in Normalised Bandwidth of a signal
You can take the FFT on any signal after modulation, but would be simplest (fewer samples required) to do it immediately after modulation. With proper upsampling the FFT of your signal prior to upsampling would be identical as long as you used proportionally more points to keep the time length identical. In your example of 100 KHz and 30 MHz, you could take a N point FFT at 100 KHz or a $300 N$ point FFT at 30 MHz and the result should look identical (with just more points interpolated in the FFT due to the upsampling). Given the 100 KHz sampling rate and room for your interpolation filter, I would assume that your GMSK modulated signal BW you are trying to view is less than 70 KHz (as a complex signal)?