Before you said "spamhole", now it's security. In both cases it makes literally no sense to block Gmail and allow random personal domains. I suppose the setup could be based on whitelisting domains, and because Gmail is so big, they won't whitelist it, and it also won't receive emails from anything random, but from specific personal domains. But such a setup specifically isn't for communicating with the world at large, and not any kind of widespread practice.

Also, you should be using your own domain with Gmail (or any other provider) so you aren't tied to anyone, anyway.