I have been using an Outlook (desktop) plug-in called gSyncit to accomplish a bi-directional Outlook <==> Gmail calendar sync. The plug-in is created/supported/sold by Fieldston Software and costs $20.
It can sync a lot of other data besides Google Calendar with Outlook (I also sync Outlook and Google contacts). I've been using the product for about 5 years and it gets the job done. Their product support people are knowledgeable, responsive, and US-based. A new version is released every couple of years, usually to accommodate new protocols required by Google.
I've looked for other (and free) ways to do this, and haven't found anything better than gSyncit.
Good luck,
Michael