Gmail Notification stopped working on Nexus 4
On stock Android 4.2.2 on my Galaxy Nexus I had the same problem of gmail notifications not working. I checked that in Settings>Data Usage "Auto Sync" was turned on. 

Then I went to Settings>Accounts>Google, hit the email address and you see everything that is being synced or not. 

Weirdly for me it Gmail sync was inactive after I had recently updated my rom. 

This should take care of this problem.