Hello !
Unfortunately, at the moment, you cannot use Google Calendar as your primary calendar in Outlook for the web.
You can add your Gmail account to the desktop version of Outlook (new) to use your Google Calendar, but completely deleting the Outlook calendar is not possible.
If you're using classic Outlook, synchronization is only in one-way : your Google calendar events are displayed in Outlook, but you won't be able to add other ones.Please let me know if you have any other question !