Well I’ll be damned. I think we unfirewalled push-test.mattermost.com and now it works with your server.
So that’s a good news. But since you advertise your test server may not be stable enough, I would like to make hosted push working.
This is the documentation I followed:
https://developers.mattermost.com/contribute/mobile/push-notifications/service/
and it seems to be working (curl test) to the point, where push server talks to google services/api
but google does not like all the info supplied.