The article that you linked shows how to "Allow Gmail to receive emails using POP3". That is to say, your mail should already be hosted elsewhere, and you'd be using the Gmail web app as a client for your email hosted on a different server.
Yes, Google Domains allowing for email forwarding sounds like a reasonable feature.
But when it comes to sending mail, the support pages say:
> While these directions let you send emails from a custom email alias at your domain, email recipients can still find your personal Gmail address if they inspect the email headers.
To me, this is no replacement for a mailbox on my own domain.