This solution should solve the "Error 403" issue.  And this terrible user interface keeps giving it's own "403" error whenever I try to paste an actual URL, thus all the inserted spaces below.

I assume you already got to the last step of these instructions:
https : // www . odoo . com / documentation / 16.0 / applications / general / calendars / google / google_calendar_credentials . html

Now you need to open the OAuth consent screen:
https : // console . cloud . google . com / apis / credentials / consent

OPTION A: KEEP AS TESTING
Scroll down to "Test users", click the button "ADD USERS", enter your same e-mail address that is used for your Google calendar, be sure it is exactly correct because you are limited to 100 test users and can never delete one once added.
__OR__
OPTION B: PUBLISH TO PRODUCTION
Under "Publishing Status" it will say "Testing", click the button "PUBLISH APP" and follow instructions.
Now the screen should update to say "In production" instead of "Testing".

After choosing option A or option B above, retry the failed 403 authorization, it will ask you for permission multiple times and may give you a warning that the app is not verified, you will have to click "Advanced" and "Continue to Odoo (unsafe)".
After several authorization steps, you should end up back at your Odoo calendar, and will now be properly authorized to sync with your Google calendar.

I got my info from here:
option A: https : // www . odoo . com / documentation / 16.0 / administration / maintain / google_oauth . html
option B: https : // wordpress .org / support / topic / error-403-access_denied