Not sure if this will be helpful, but here is my experience:
I installed Brave a few days ago to replace Chrome (I also uninstalled Chrome).
I can log into my Gmail account fine… but the mobile phone (HTML) view is displayed.
Here is the sample URL when I log in and the HTML view is on the screen:
https://mail.google.com/mail/mu/mp/719/#tl/Inbox
If I remove
mu/mp/719/#tl/Inbox
And refresh the screen, then the regular view displays, and my URL looks like this:
https://mail.google.com/mail/u/0/#inbox
This is the only fix I have found that works.