I've searched around and I haven't seen this addressed, so I figured I'd post it here since I have an EVO.
For some reason, my Gmail app stop notifying immediately upon receiving a new e-mail. I would have to manual sync in order to receive a new e-mail. I had made sure auto-sync was checked as well as in the app itself (I even unchecked/checked just in case the OS wasn't reading it). That did not help and restarting didn't help either.
Search through other forums I found this solution which worked for me.
1. Press your phone's Home button, then press Menu.
2. Select Settings and then Applications
3. Select Manage applications.
4. Click Menu > Filter > All apps
5. Select Gmail Storage and then select Clear data. Answer OK when prompted.
6. Return to the list of applications by pressing the Back key
7. Select Gmail and then select Clear data. Answer OK when prompted.
8. Restart your phone.
(credit alChemistar @ google support forum:
http://www.google.com/support/forum/...c56660b3&hl=en)
Once I got it working again, I experimented to see why it stopped working (killing the gmail app this ATK), but it continued to work.
Any one know who has experienced this know why it occurs?