Before diving into the exact how-to, we need to address an important question. 
Why embed Google Calendar in the first place? Why not use the Notion calendar itself? 
Sure, Notion is an amazing app. But its calendar feature is severely lacking, to put it mildly. If someone were to do a calendar Olympics, Google would win, hands down. 
Technically, the Notion calendar is just a database with a calendar view. Google Calendar has several important features that Notion does not. 
Email integration-   Google lets you add events to your calendar right from your email inbox. If you schedule events from Gmail, this feature is essential.
Cross-calendar integration and collaboration-   Google Calendar integrates with external calendars and lets you collaborate with others and even share calendars. 
Recurring events-     Google Calendar lets you create recurring events with just a few clicks. For example, think about your weekly team meetings. You just need to set up a reminder ONE time and make it repeat every week. 15 seconds and it’s done. On Notion, however, you would have to do this manually every time unless you set up a recurring meeting automation with external tools. 
So, embedding Google Calendar in Notion would let you enjoy the best of both worlds!