The logs indicate that the message is being delivered for a gmail.com address successfully. You have left the default configuration in config.inc.php, where OJS will use PHP’s mail() function to send the email as the sending user. If the sending user doesn’t have an address which is hosted on your domain, of if the domain is not configured via SPF and DMARC to allow mail sending, then this will likely be flagged by Gmail as spam.
See this discussion for more info: