Basically, subscribing to the Google calendar with the Outlook Web App is very similar to subscribing to the desktop app: first, of course, you have to find out the necessary iCal address of the Google calendar before logging into your Outlook account. There you switch to the calendar view as with the desktop client and click on "Add calendar." Then, select "From the Internet" from the options above.
In the next field, insert the iCal URL of your Google calendar and specify the desired name.
Now, you can retrieve your subscribed Google calendar at any time and also view the appointments entered there in Outlook. However, you cannot enter your own appointments in a subscribed calendar.