While the X2Go client (package "x2goclient") can be downloaded from the main Ubuntu repository; this is not true for the X2Go server. One needs to get it from a different repository by doing the following (according to http://wiki.x2go.org/doku.php/doc:installation:x2goserver):
sudo add-apt-repository ppa:x2go/stable
sudo apt-get update
sudo apt-get install x2goserver x2goserver-xsession
I understand X2Go has been around for some time. How come it is not entirely available from the official repo, and the server component is available only as a PPA?
As fkraiem says, it's because nobody packaged it for Debian. And in Debian, it seems a few people expressed in intent to package it, but never followed through. See Debian bug report 465821 for more information (it's been sitting around since 2008).