Based from my own troubleshooting:

Can confirm this issue is not related to 73.0.3856.431 as I was having this issue with 428

It may or may not be related to the recent Windows Security Updates , it could just be a coincidence but I did start getting this problem after installing the following:
https://support.microsoft.com/help/5001330 KB5001330 , KB5001405

Workarounds:

Option 1: Login with Chrome, Firefox will allow Gmail to temporarily work in Opera
Option 2: opera://settings/system enable "Use DNS-over-HTTPS instead of the system’s DNS settings" only worked for me with Google Public DNS selected.

Will be uninstalling the Security Updates to test if they are causing the issue or not.