Official packages for Debian:
Newer / more packages for Debian provided by X2Go upstream:
Newer client versions and X2Go Server + Add-Ons are available in X2Go upstream's PPA(s) on Launchpad. The team creating and updating packages for Ubuntu and its derivates is found here: https://launchpad.net/~x2go
Here's an overview of the current PPAs for Ubuntu and derivates - this list is also found on the teams Launchpad page.
Follow the instructions given on the different PPA pages on how to add the PPA of choice to your system.