gSyncit is a Microsoft Outlook addin that allows for bi-directional synchronization between Google and Outlook calendars. Once installed gSyncit adds a toolbar to Outlook and offers the following fearures.
1. Simple and easy-to-use interface and configuration
2. Synchronize Google to Outlook calendars.
3. Synchronize Outlook to Google calendars.
4. Synchronize events within a predefined range date or day range
5. Collision detection alerts when an appointment has been modified in both Google and Outlook.
6. Automated synchronization
7. Provides access to public and private Google calendars
8. Provides two-way sync for private calendars
9. Provides one-way event publishing for public calendars
10. Provides support for publishing free/busy data only
11. Matches duplicate entries when performing an initial sync between Google and Outlook
12. Works with Outlook 2003, 2007 and Microsoft Exchange accounts
13. Supports accessing multiple Google calendars
14. Supports access through proxy servers

The license of this software is Free Trial Software, the price is $14.99, you can free download and get a free trial.