Yet another updated answer with little more explanation for the confused(..like I was).(with due credit to @azgolfer, @PankajAndroid and @Adam)
1- If you create a project at https://console.developers.google.com, you can see two things "PROJECT ID" and "PROJECT NUMBER" in project overview. Project Id is no longer numeric but alpha numeric like master-xxxxx-1234. Project number is a 11 digit number like 123456789100.
2- Even when you inspect the browser link you can find it as:
https://console.developers.google.com/project/master-xxxxx-1234
So is master-xxxxx-1234 is the SENDER_ID (at some places referred as PROJECT ID)? NO.
You have to use the project number as project id in GCM.
3- Where can you cross check that project id is being used as project id?
On API Dashbaord. (url: https://code.google.com/apis/console/b/0/?noredirect#project:12345678900 - where 12345678900 is your project number on project dashboard.
SUMMARY: Project Id in Project Dashboard and API Dashboard are different. The project number in project dashbaord is used as project id in API Dashboard and Project Id in project dashbaord as Project name in API Dashboard. So the project number shown in the Project Dashboard when we create project in developer console is used as SENDER_ID in GCM.