All of our appliances include Postfix and as per all of our appliances, it should be configured to send emails directly (i.e. via 'sendmail').

In theory that should work fine, but in practice is very hit and miss. (Often IP addresses are blacklisted, etc).

You can configure your server to use a remote SMTP proxy via Confconsole << Advanced << Mail relay. (Please note that the Confconsole Let's Encrypt plugin is not compatible with GitLab).

After configuring that should get both system emails and GitLab emails.

Alternatively, you could configure GitLab to use an alternate config (e.g. configure that to use an SMTP relay directly).