• Download Google Calendar Sync, offered by Google. It allows you to sync events between Google Calendar and Microsoft Outlook Calendar. You’ll be able to determine the direction of information flow, as well as the sync frequency.
• Sync your iPhone calendar events with your Outlook calendar.

That’s it!

Google Calendar Sync offers three types of Sync Options between Google Calendar and Microsoft Outlook Calendar.

• 2-way sync:
  All events in your primary Google Calendar and your default Microsoft Outlook calendar are synced together.
• 1-way: Google Calendar to Microsoft Outlook calendar
• 1-way: Microsoft Outlook calendar to Google Calendar

Limitations
Google Calendar Sync will only sync events from your primary Google Calendar and your default Microsoft Outlook calendar at this time.

Other solutions

OggSync
OggSync is feature rich, for more advanced Google Cal users.
It supports Multiple Google Calendars and Multiple Outlook Calendars
Import Google Colors, Google Calendars mapped to Outlook Categories etc

However, OggSync is not free. I have not tried it yet (38MB installer? Huh!)