FYI: We know that it is a SenderID issue as we modified push-proxy to give us some diagnostic information, and when we directly call it with the Mattermost ServerID and DeviceID, GCM accepts the message, but gives us the error:
2016/04/12 21:02:17 ERROR Android reponse: &{5356772378128812979 0 1 0 [{ MismatchSenderId}]}
Which means it liked our AndroidApiKey, but the DeviceID (generated in part from the ServerID) didn't match the approved Project ID.