My understanding is that GMSK is just Frequency Shift Keying.  The acronym stands for Gaussian Minimum Shift Keying. The minimum shift part means that the data and carrier are synchronized so that the data changes happen when the carrier is zero. This removes phase discontinuities. The data is run through a Gaussian filter before it is transmitted.  The main objective is to reduce the sideband power. This makes it a very power efficient mode and it is part of the GSM Mobile Phone standard. It is a good choice for something battery powered, like a phone or a small spacecraft. With that said it is harder to decode than straight FSK with a Root Raised Cosine Filter.  So it might be better with strong signals than weak ones. That is just my conjecture. I'm sure there is bit error data somewhere that can answer that question. 

73

Chris 

G0KLA / AC2CZ