The problem here lies with your Return-Path. We provide a configuration option that can force the envelope sender to an address that is represented by the SPF record your domain. In your case, your server is sending an email and looking like a Gmail email address when your server’s IP is not in the SPF block for Gmail.
There is a commit in pkp-lib that will be in a later version of OJS that addresses this:
https://github.com/pkp/pkp-lib/commit/7af3bf24a2e24f8e2f4afcde4750d7b16d9b80f7
You can try to roll that into your install now if you like. Alternatively, use a different email address than a gmail.com address, or configure OJS to use SASL or email authentication and send mail that way.
Regards,
Jason