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:

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/google_apis/gcm https://chromium.googlesource.com/chromium/chromium/+/trunk/...