I changed my password now Mail will not connect to gmail. Error 400

You may need to set up an app specific password. Open your gmail in a browser and there should be an option in the settings where you can specify Apple mail as the client needing the specific password, copy the password and use it to login to gmail on Apple mail.

The Error 400 problem is with Google Gmail not your iMac.

see > HELP! Mail app on iMac failed login with … - Apple Community

and > Can not add my Gmail account to my MAC - Apple Community

and > ''Sercannot process the request because i… - Apple Community