Message delivery to gmail can fail due to multiple reasons, most often it's due to missing SPF, DKIM and DMARC records relating to your domain. If these records are not in place, you will almost always immediately get a non delivery report (NDR) in your inbox. If your records are in place and still mail delivery is not happening, check whether your mail has gone to the SPAM folder in Gmail. A proper PTR record could correct that issue permanently. If all the above are in place, then you may have to look at your MTA logs. If you are using Postfix, check /var/log/maillog to investigate further.