GMail on one Mac (only!) stopped working this week
Do you have your account configured with your regular GMail account password or an "App Password"?  Do you have two-factor authentication turned on? 

I've had my home cameras configured for several years to send me emails via SMTP using my GMail with an App Password to email me if any motion is detected. I usually get several emails a day but this week suddenly it stopped working.  I generated a new Google App Password and even disabled 2-factor authentication but I still can't send any emails from my cameras.  Again... this has worked flawlessly for 2-3 years with no recent changes to my camera.

I encountered someone else having similar issues here: https://www.cnet.com/forums/discussions/outlook-365-no-longer-connects-to-gmail-smtp/

I'm thinking google must have changed something.