It isn’t FreePBX that is doing this, it is the OS. It will actually just be mailing it to the local user that owns the crontab, i.e. asterisk, and the system mail program is adding the domain name and trying to delivery using Gmail. On simple systems, it would have just ended up in /var/spool/mail/asterisk.
Getting FreePBX systems to send mail addressed to local users, to a completely different email address is discussed in: