How do I replace the "From" address from my mailings?
So I am assuming that the only thing that changed between "it worked" and "it doesn't" is to switch from your own smtp to your gmail one.

Beside the problem you mention, using smtp from gmail will not work to send mass emails, they have a lot of limits and as soon as your list has more than a handful of emails, you will have problems.

There as several providers (sendgrid, mailjet...) that are integrated with civi, and I'd recommend you switching to one of them

And it should solve out your gmail.com problem too