I was expecting this to be "why" as a policy matter, but instead it's a technical description (mostly about the fact that SMTP is different from HTTP, for people who might not appreciate that).

I guess the idea may be that Chinese people are not supposed to have or use Gmail accounts, but are allowed to communicate with other people who do. (This isn't necessarily a super-bizarre distinction to imagine a government making; the cost of not being allowed to exchange e-mail with anybody on Gmail is really quite high.)

Edit: also, people have told me that there is a legal restriction in China on the ability to listen on (some?) TCP ports or allow a hosting subscriber to do so (!) -- you're supposed to have a government license to have a TCP listener and your hosting provider or ISP can be punished for allowing you to operate one if you don't have the license. That means that Chinese Internet users couldn't easily set up their own personal mail servers free of government oversight and surveillance. So they can write to Gmail users all they like, but it may be trickier to receive a reply except via a government-approved mailserver.