In Gmail, at the bottom of the screen if you click Details it will show you your recent account activity and there's a button to sign you out of all other sessions. How does this work? Are they somehow revoking cookies from the server side?
This is a pretty simple task to accomplish.
Basically the server will store the session IDs of each individual active session in a database somewhere. Revoking a particular session is a simple task of removing the record from the database, which will invalidate the session.