Hi there, I am Cris and will be glad to assist you.
Do you have access to your Gmail account? Meaning are you able to sign in with the correct email and password ***** Gmail.com?
Have you tried to reset the password?
No to what?
Again, on your new computer have you gone to Gmail.com and entered the email and password ***** in successfully?
If you answer no, then that means you do not know your password. On your iPhone the the password ***** ***** so you don't enter every time. Same thing on your old Mac under the mail application.
If you don't know your password ***** will need to reset then you can update it on your iPhone or any other devices..
Did you reset your Gmail password? Or would you like for me to provide you the link?