You need to set up your Google Mail as an Exchange account. First either delete or turn off all the syncing in your normal GMail account settings. Then follow these instructions to set the account up again as an Exchange account. Don't worry, you'll lose nothing. It all stays on the server.
Setting up Google Sync with your iOS device : Google Sync (ActiveSync) - Google Mobile Help
Once you've done that visit m.google.com/sync on your iPad. Once you sign in you'll see your iPad listed as a device, select it and you'll see a list of all your Google calendars, including the subscribed ones. Check to sync.
As a bonus you'll be able to sync your Google Contacts to the Contacts app.