There are a lot of considerations in ensuring your messages are not flagged as spam. I discuss some of the technical considerations here:
My advice on the bottom of that post is your next step. Identify what part of email standards (SPF, DKIM, DMARC) is creating your spam problem, and you should be able to address it.
On the other hand, if you are sticking with Gmail as a sender, a shortcut for you might be to try setting config.inc.php’s force_default_envelope_sender to your Gmail address, and to use a proposed patch which will reintroduce the reply-to functionality.