[Aissen]

Maybe, since this hits Google APIs directly: https://github.com/microg/android_packages_apps_GmsCore/blob...

But then, there's no TOS at this URL, and there's no point in taking this repo down, it would only make Google look bad and give microG free advertisement.

[larma]

Most GCM and checkin related code is actually a Java rewrite of open-source code by Google. They released a client and some protocol specs as part of Chromium: https://chromium.googlesource.com/chromium/chromium/+/trunk/...