Mac Mail and Gmail - Error 400: invalid_request

 Mac's Mail App is not as modern as iOS's Mail app, Many eMail providers now require a separate App Password different from your eMail password…

It may just need an App Password separate from the gMail password…

https://support.google.com/mail/answer/185833?hl=en-GB

Apple Mail won't sign in to Google account - Apple Community

https://discussions.apple.com/thread/254695620?answerId=258765671022#258765671022

my iMac has OS Yosemite 10.10.5 Does this… - Apple Community

gmail on mail Version 11.5 (3445.9.1) - Apple Community

Note from PrestoWebDesign…

Trying to add Gmail account to Apple Mail… - Apple Community

no one is getting the answer to why error 400 in mac mail keeps stopping one adding gmail or the g mail stopped working. trying to resolve Apple help got me to delete the mail to re create it and in doing so I lost all my gmial s m now in **** street as I cannot access two of them as only two other fit on my phone . I need them to work on my i mac and mac book . For work flow. I cannot up grade as programs i use are not supported only by High Sierra.they are both 2011 just up graded HD to SSD s I cannot afford a new mac in any way .

Some one lease resolve this issue or get google to fix it.

Good work, great news my friend! :)