How I got it to work on my Emacs 28

[quotuva]

In the gmail console I had to set the scope to https://mail.google.com/ Lines 202-212 are already part of Emacs 28, and seemed to be causing problems - so I removed them. auth-source-xoauth2-creds set to a file didn't work (auth-source-search tried every combination and came up empty handed, even when I tried it manually without the :user field). So I had to hard code my username in line 166 because nnimap uses that default username of nil.

[stasvlasov]

Thank you @quotuva! I would have gave up trying to make it work if not your comment. Your instructions worked as charm.

[manphiz]

It looks like the reason that the user part may be nil is because one doesn't set nnimap-user in gnus-secondary-select-methods. Once you set that (e.g. to your email address) it will be used in the user field.

[quotuva]

Thanks! I've not been using this for a while now, but it's useful to know.