Yes, you read that right. I don't have any more problems with GCM on Windows 10 Mobile. None at all ! :p
Of course, that is because I broke my Lumia 650 screen, and it looks like it will either cost me too much to fix, or most likely I will fix it myself with a cheap Chinese screen, already ordered. But I won't be using it again once I have recovered all the data from it. I am converting to Android.
I'm currently using a borrowed Samsung S5 Android phone and GCM is working beautifully on it. Sure, sync sometimes fails. But quickly recovers with a second try. I can enter weights into GCM, something that never worked in the W10M version because I could never sync anything from the W10M to Garmin Connect. I can view the results of my activities in GCM on Android.
I've used GCM quite a bit today and Android is reporting that it is only using 4% of the total battery usage. Less than the screen and the same as Google Services. So that looks good.
I bought the Lumia 650 last year because my old iPhone wouldn't support the Microsoft Band 2 App. The Band died in July this year and I bought the Garmin Vivoactive HR as a replacement, in part because GCM worked on Windows 10 Mobile. But my time with Windows 10 Mobile was really only an experiment anyway. I wanted to try it before W10M died altogether, and I guess I have now achieved that. The Lumia 650 was a very cheap smartphone and worked really well for the price. But they aren't very strong, do twist and distort if you aren't careful, and the internal screen cracks easily making the phone useless.
So all you holdouts still on Windows 10 Mobile; Give it up. It is a bad joke. Microsoft should be ashamed of themselves for letting it drag on so long. But they excited with minimum cost to themselves, and maximum disruption to their customers instead. They can't even fix my Lumia! They have outsourced services for it, and the service company outsources to a repair company that just closed business. I had the same trouble with the Band; No support personnel so Surface people handled it, no parts for repair, no stock for a replacement. I got a full refund after a year instead. The Lumia is still under warranty, but I do not expect them to fix it, as they don't seem to have fixed anyone else's cracked screens, even when the user was not at fault.
Microsoft doesn't have your interests as a priority, so don't return the favour by being loyal. I will now be buying an Android tablet instead of a Surface tablet, and I will start thinking about Linux as an alternative to Windows in the future. Bye bye Microsoft, if I can, and as soon as possible.