Work for me by adding this in the nginx configuration: 

• proxy_set_header X-Forwarded-Host $host;

Inside location /

Whilst creating credential for your calendar app and setting up OAuth Client ID. Under the Authorized redirect URIs section, after you click + Add URI and when type your company’s Odoo URL address, use http rather than https.

odoo 16

same error, 

i modify  "web.base.url"  http ---> https

but no work.

I came across a similar issue. Resolved the issue by adding the following "Authorized redirect URIs" to my project in google's cloud console. 

https://mycompany-example.com/google_gmail/confirm

There are many reasons that odoo fail with proxy. I try to show you some cases.

Enable Proxy mode

 First of all, check the proxy mode is enabled. This option tells odoo to configure werkzeug with XFH headers.

In configuration add the following option

proxy_mode = True

and in the command line

... --proxy_mode=True

Add XFH headers

The wekzeug process XFH and update request. So it is very important to add headers in the proxy (such as Apache or Nginx). Here is the list of XFH headers:

• X-Forwarded-For 
• X-Forwarded-Proto
• X-Forwarded-Host
• X-Forwarded-Port
• X-Forwarded-Prefix

Note that, if the X-Forwarded-Host is not in headers, then the proxy fix process will be ignored by Odoo (I`m sure in Odoo 14). In my case, Nginx did not set X-Forwarded-Host properly, so everything went wrongs.

THIS WORKED FOR ME (THX VMCH):

Make sure system parameter "web.base.url" set with https 

Settings/Technical/Parameters/System Parameters

Problem solved, thank you very much