This is almost always the sender, not the form. Three things to check:

1. Global Configuration → Server → Mail: use SMTP with an account on your own domain instead of PHP mail. The "From email" must be an address on the same domain.
2. DNS: the new host's mail server has to be in your SPF record, and DKIM should be switched on at the host. Without them Gmail treats the mail as spoofed.
3. In RSForm, the admin email's From should be that same domain address – put the visitor's address in Reply-To instead.

Then send a test with "Send Test Mail" in Global Configuration and look at the headers in Gmail (Show original): SPF and DKIM should both say PASS.