CryMP has own GameSpy master server emulator available at m.crymp.net that provides only server list. Other GS services (like profiles, natneg, cd-key validation, or lobby chat) are not implemented, but most of them don't make much sense today anyway. Especially GS profiles are big problem because they are not so secure. CryMP has own secure profile system that uses just HTTP requests.
You can still use any other GS master, but your server won't be visible on the in-game server list, so it's quite useless. :) You can change GS master in-game using cl_master cvar (added by CryMP client mod). Changing its value takes effect immediately, just refresh server list. Similarly, you can change GS master on server using sv_master cvar, but it's available only in SSM SafeWriting. You can also use modified CryNetwork.dll file or "hosts" file to change GS master server, but these 2 ways are not so nice.