I vaguely suspect that GP may refer to the fact that certain corporations block access to the Gmail web interface (and the other big providers) from their network, typically for auditing purposes, in a way to a personally run mail server might evade. In those cases, actually accessing your personal mail server would typically be a gross breach of conduct and lead to immediate termination for cause if caught. This, of course, has nothing at all to do with the deliverability of email from those services, even to these same organisations (they can audit what goes through their own servers, so that's fine).
But, that's a guess, and I obviously can speak on GPs behalf.
Yes and also they block receiving emails from GMail domain for security purposes. So if you want to email someone internal in big co, you have to do it from your own domain. I suppose it is not only for GMail. Probably Yahoo, Microsoft also but I only had one guy trying to use GMail sending mail to corporate users.
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.
Any sane corporate security? You can setup gmail account in 5 mins and start phishing people in corporate env. That is why receiving emails from generic domains is no go. I also had actual requirement where we did not allow people to register with gmail account, only non generic domain emails allowed. So that you use your company email for B2B product.
