You need to use an app password now. Google has made some changes and Gmail no longer accepts messages from "less secure apps", which used to work with Calibre.
Here are the instructions for enabling app passwords in your gmail account.
https://support.google.com/mail/answer/185833
Note that you must turn on 2-step verification first. If you are unable to turn it on, then you may need to create a separate gmail address for use with Calibre.