If you are on an iPhone, you need to add your Google account to the calendar…
1.First, make sure your Google calendar is synced. The process is the same on all devices.
2.Go to Settings. Then open Mail / Contact / Calendars.
3.Next, select Add Account and Gmail. If you have already added your Gmail account, then instead of Add Account, choose Gmail from the list.
4.Toggle the Calendar switch to On