See the Stack Overflow post Why do I get "MismatchSenderId" from GCM server side?.
From the URL above:
double check the Sender ID and API_KEY, they must match or else you will get that MismatchSenderId error. In the Google API Console, look at the URL of your project:
https://code.google.com/apis/console/#project:xxxxxxxxxxx
The xxxxxxxxx is the project ID, which is the sender ID.
Some people are reporting issues using the "Key for Server Apps" type of keys, but having success using the Browser Key type instead. Personally, the Server Key type works for me, but try both, YMMV.
