Currently Gmail default settings on iOS4 devices won’t work as Push mail.
So you have to setup a your Gmail account as an Exchange Server and push mail notification will start to work correctly.
1. Open the Settings application on your device’s home screen.
2. Open Mail, Contacts, Calendars.
3. Tap Add Account…
4. Select Microsoft Exchange. Enter Account Info.
5. In the Email field, enter your full Google Account email address.
6. Leave the Domain field blank.
7. Enter your full Google Account email address as the Username.
8. Enter your Google Account password as the Password.
9. Tap Next at the top of your screen.
9a. Choose Cancel if the Unable to Verify Certificate dialog appears.
10. When the new Server field appears, enter
11. Press Next at the top of your screen again.
12. Select the Google services (Mail, Calendar, and Contacts) you want to sync.
13. Unless you want to delete all the existing Contacts and Calendars on your phone, select the Keep on my iPhone option when prompted. This will also allow you to keep syncing with your computer via iTunes.