GNU Emacs Gnus は Gmail IMAP に接続できません

IMAP経由で私のGmailアカウントからメールを検索するようにGnusを設定しています。アドバイスとしてやってみましたhttps://www.emacswiki.org/emacs/GnusGmailしかし、起動時にGnusで次のエラーが発生し続けます。

Opening connection to imap.gmail.com via tls...
nnimap (gmail) open error: 'NO (ALERT) Please log in via your web browser:
https://support.google.com/mail/accounts/answer/78754 (Failure)'.  Continue? (y
or n)  y
Saving file /home/mark/.newsrc-dribble...
Wrote /home/mark/.newsrc-dribble [2 times]
Gnus auto-save file exists.  Do you want to read it? (y or n)  y
Opening nnimap server on gmail...
Server nnimap+gmail previously determined to be down; not retrying
Opening nnimap server on gmail...failed: NO (ALERT) Please log in via your web
browser: https://support.google.com/mail/accounts/answer/78754 (Failure)
Checking new news...
Reading active file from gmail via nnimap...
Opening nnimap server on gmail...
Server nnimap+gmail previously determined to be down; not retrying
Opening nnimap server on gmail...failed: NO (ALERT) Please log in via your web
browser: https://support.google.com/mail/accounts/answer/78754 (Failure)
Reading active file via nndraft...done
Checking new news...done
No news is good news
Warning: Opening nnimap server on gmail...failed: NO (ALERT) Please log in via
your web browser: https://support.google.com/mail/accounts/answer/78754
(Failure); Server nnimap+gmail previously determined to be down; not retry\
ing; Opening nnimap server on gmail...failed: NO (ALERT) Please log in via your
web browser: https://support.google.com/mail/accounts/answer/78754 (Failure);
Server nnimap+gmail previously determined to be down; not retrying
gnus-group-read-group: No group on current line

GnuTLSがインストールされており、gnutls-cliがimap.gmail.com：993で動作しているようで、「OK Gimapが要求可能です」というメッセージが表示されます。

これは私の~/.gnusです。

(setq gnus-select-method
      '(nnimap "gmail"
          (nnimap-address "imap.gmail.com")
          (nnimap-server-port 993)
          (nnimap-stream ssl)
          (nnir-search-engine imap)
          (nnimap-authinfo-file "~/.authinfo")))

(setq smtpmail-smtp-service 587
      gnus-ignored-newsgroups "^to\\.\\|^[0-9. ]+\\( \\|$\\)\\|^[\"]\"[#'()]")

私の~/.authinfoは次のようになります。

machine imap.gmail.com login [email protected] password my_password port 993
machine smtp.gmail.com login [email protected] password my_password port 587

何が問題なのでしょうか？