I had the really same issue with my Fenix 5 sapphire after upgrading to Android Pie on my Xiaomi yesterday.
But I was able to fix it following the informations given in this thread :
https://forums.garmin.com/forum/into...id-pie-problem
The problem doesn't comes from GCM, but the way we have to pair the device and the phone under Android 9.
It seems that we have to pair it with the phone, before going into GCM app.
Today, everything is fine. It worked fine during my morning run of ~50 minutes, then I turned it off, and sync'd via cable. Disconnected and turned watch off. Turned it back on by charging via cable connected to my laptop (different computer). Activated Garmin Connect on my iphone. Tried several setting changes and faked a strength workout. No problems so far. ??? IOS is not updated, and watch is still at 12.0.
I had a chat with Garmin support unfortunately it seem we are the only one with those issues on Android 9. I thinks it is from the Xiaomi and the software. I am tired of looking for a solution at the moment and hope a miracle will happen :D
But this is pure Android, there is no additional software...
I found in internet other threads for Bluetooth connection problems similar as ours with Android 9 and other smart phones.
Yesterday I sent a bug report to Xiaomi through the phone for this problem and I hope that they will check it...
You can do this too - find the "Feedback" icon in the phone apps. The more users report the bug, the more likely it is Xiaomi to do something :)