If you are paid-up Google Apps user (Business or Educational) you can download a sync tool that will sync your Google Calendar with Microsoft Outlook and provide full read/write access.

If you are free Gmail user, you will have to look for a third party tool or do with a read-ony access in your Outlook.

To view Gmail calendar in Outlook (read-only):

• Open Gmail Calendar
• Go to Tools > Settings
  
  
• Click on Calendars tab, then click on the calendar you want to view (it’s normally your name)
• Down the bottom, click on ICAL next to Private Address and copy the link displayed.

• Now open Microsoft Outlook.
  I’m using Outlook 2010 in this example but similar process applies to other versions
• Click on File > Account Settings
• Switch to Internet Calendars tab and click New
  
  
• Paste the address you copied previously, name your calendar and adjust other settings as required

Now you will be able to view your Google Calendar in Microsoft Outlook.

Windows 7
MS Outlook 2010
Gmail
06.2013