Log in to your Google Calendar account.

In the left column, click My calendars to expand it, hover over the calendar that you want to add to Outlook as an Internet Calendar Subscription. Select the three dots to the right of the calendar name, then select Settings.

Scroll down to the Integrate Calendar section and right click on Secret address in iCal format and select Copy. 

In Outlook, click File > Account Settings > Account Settings.

On the Internet Calendars tab, click New.

Paste the address you copied in step 4, by pressing CTRL+V, and click Add.

In the Folder Name box, type the name of the calendar as you want it to appear in Outlook, and then click OK.

When you start Outlook, it checks Google Calendar for updates and downloads any changes. If you keep Outlook open, it also periodically checks for and downloads updates. (Google Calendar tells Outlook how often to check for updates.)