when "Secured connection using TLS" is selected, adding the account
failed with the message "Couldn't reach server. Please double-check the
server and port number."

this might be caused because the option makes Gmail attempt StartTLS,
which is not available on port 465 on the submission server: when a
connect attempt is made when Gmail tests the account, "SSL_accept:error
in before SSL initialization" is seen in the postfix logs

When "SSL" mode is selected, adding the account and sending mail using
it works as expected

service/email.md

@@ -141,7 +141,7 @@ little different.
     - Set "Port:" to 465.
     - Set "Username:" to your username (_without_ "@torproject.org").
     - Set "Password:" to the email submission password that you configured.
-    - Select "Use TLS", not "Use SSL".
+    - Keep "Secured connection using SSL (recommended)" selected
 
 Double-check everything, then click "Save Changes".  Gmail will try
 authenticating to the SMTP server; if it's successful, then the popup