Gmail only allows modern authentication by default, MS will enable this by default later on this year.
It is best to pro-actively enable this, to prevent the future disruption of of accounts disconnecting.
First check to see how many applications this will affect:
- Navigate to the Azure portal > Azure Active Directory > Sign-in logs.
- Add the Client App column if it isn't shown by clicking on Columns > Client App.
- Add filters > Client App > select all of the legacy authentication protocols. Select outside the filtering dialog box to apply your selections and close the dialog box.
- If you've activated the new sign-in activity reports preview, repeat the above steps also on the User sign-ins (non-interactive) tab.
Next enable Modern Auth across the organisation.
In the Microsoft 365 admin center, go to Settings > Org Settings > Modern Authentication. In the Modern authentication flyout that appears, click to enable or disable Turn on modern authentication for Outlook 2013 for Windows and later (recommended).
This In order to connect an account using Modern authentication you need to delete the old Basic account first.