I am not sure which client you are using but from a search:
“The mail server is rejecting your email because you’re trying to send from a username which differs from the username you’re logging in as, e.g. you’ve set the From: to be no-reply@mydomain.com , which is obviously not the same as admin@mydomain.com”
Which I would guess (really I am guessing) might mean you are logged in with your regular gmail account while trying to send from the new gmail address you have created.
make sure you are logged out of your old identity completely…
or by identity do you mean an alias for your regular account?
You could try; leave the “From” field to autofill. At the right on this field is a “drop down arrow”. Click this twice to reveal the " Reply to" field.
I did not know email servers still let you do that. I was working for a provider some years ago and our servers would not let clients do that at all… I am pretty sure many, if not all, of the larger providers would not allow it unless you were a business entity that had legitimate reasons for doing so and a rule was provided especially for you. Address spoofing can be used for all sorts of nasty things.