Please try this (I'm not sure it's reason why it's working for me now, but I did it and now I can sync custom strength)
1) delete all custom strength workout from GCM and Sync via bluetooth
2) clic to create strength workout, and save it without any modification.
3)open this workout and send it to fenix5
4) sync your fenix 5 via Bluetooth
5) in my case,strength workout appear for the first time on my fenix5
6) It seems I can create and sync custom workout via GCM after that.