@rebs Gmail will send to fail if any part of the formula is wrong, not just the recipient. Do you have everything else working? I would ‘hardcode’ your email address in as the recipient “rebs@email.com” and try to get it to send. If it does, then try to change the recipient to a formula so you can troubleshoot.
Can you share any screenshots of your rule? Often times the rule must also be set to run as yourself (who has access to the gmail pack) rather than being run by automation bot (which is default).