If you don’t feel like installing Microsoft.net framework or shelling out money for an Outlook/Google Calendar syncing app, try the low rent method.
1. Create a new Gmail account, and a Calendar account with that address
2. Share that new calendar with your current Gmail address
3. When you’re setting up any meeting in Outlook, just cc the new Gmail address
4. The appointment will automatically show up as tentative in the new calendar.
5. If you’re making an appointment in GCal, just do the same, in reverse: include your Outlook email address. The appointment will automatically show up as tentative in your Outlook calendar until you accept.
6. If you’re accepting a meeting, just forward that request to the appropriate email account for automatic tentative adding.
Sure, this isn’t as elegant as an automatic sync, and it doesn’t handle moving of appointments very well, but it’s a great low rent solution that would probably also work for iCal, Thunderbird, etc.