Finally, Google has enabled the feature to Sync Multiple Calendars through Google account to Windows Phone devices. At the moment official support is provided to sync 25 Google calendars to Windows Phone. Through guide below you can learn the process to do so, which is quite easy.
How to Set up Gmail in Windows Phone 7
Sync Multiple Google Calendars with WP7
On your Windows Phone 7, open Internet Explorer and visit http://m.google.com/sync
On Google syncing services page, “Sign in with your Google Account”
On “Manage devices” page select your Windows Phone 7 device
In “My Calenders” section check mark the boxes followed by your Google accounts and click “Save”
Confirmation screen will display and that’s it
Note: Google will take about 30 minutes to sync and add calendars with Windows Phone 7.