You may have find by yourself, but maybe for other users…
If you want to be able to access modifications of your google calendar(s) from nextcloud, you have to configure the calendar from google cloud.
From your Google calendar cloud place online, on your calendar selected options, you have to add the email of the user you want him to be able to modify the calendar, and also, define wich kind of access rules he can have.
Then, from your nextcloud application, your user has (if you define it) an email linked. This email is the one to use for give access rules to google calendar shared with him. It is a minimal protection who said what which one connected can do.
hope this will help someone.