I bought a Fenix 5X at the weekend, and paired it to GCM on my phone with no problems. It did an initial sync, was picking up my step and HR data, etc.
The next morning, I did another sync, and after the sync had completed, GCM closed completely. Restarting it did the same thing. Clearing the cache and data enabled me to pair them again, but GCM would still crash after the sync. I connected the watch to my PC and installed the v10 firmware, uninstalled and reinstalled GCM on my phone, but it still just crashes within a couple of seconds of opening.
Edited to add: Have been using a 910XT and Vivoactive with this phone and GCM with no crashing, this was a new problem once the Fenix started syncing. It crashes whether or not bluetooth is on, so it's not something in the active connection to the watch, it seems to be something with data which has already gone to the GCM app.