1. Tap the Settings on your iPad’s home screen.
2. Tap Mail, Contacts, Calendars.
3. Tap Add Account….
4. Select Microsoft Exchange.
Enter Your Account Info
5. In the Email field, enter your full Google Account email address.
If you use an @googlemail.com address, you may see an “Unable to verify certificate” warning.
6. Leave the Domain field blank.
7. Enter your full Google Account email address as the Username and your Google Account password.
8. Tap Next at the top of the screen. Choose Cancel if the Unable to Verify Certificate dialog appears.
9. Enter m.google.com in the new Server field.
10. Tap Next at the top of the screen again.
11. Choose the Google Calendar services to sync.
To delete all the existing Calendars on your iPad, uncheck the “Keep on my iPhone” option.
12. Synchronization will begin automatically if you have Push enabled on your device. Wait seconds to start a sync for the default calendar.
Enable Google multiple calendars
13. Goto http://www.google.com/calendar/iphoneselect. Login if required.
14. A web page opens with checkboxes for each of your calendars.
15. Select your calendars for sync’ing. DONE!
Thanks to http://apple-ipad-tablet-help.blogspot.com/2010/03/how-to-sync-google-calendar-with-ipad.html
for describing the first steps.