Single Player - doesn't continue when logged off?

To clarify: G-portal is one of the options, but it’s not the only one. And as some of us pointed out, you can also host it yourself, by running the dedicated server launcher on a computer under your control.

If your machine is beefy enough, you can run the server and the game on the same computer, and just leave the server running when you’re not playing. (Of course, if you turn off your computer, your server will also stop running.)