I asked nomachine and here is their actual license.

As I'd suspected the GPL components that they use NoMachine points out were GPL already
and nearly 80% of the overall NX project.  

Given that I'm not sure they can copyright those modules as they were based on other GPL projects... see URL: http://www.nomachine.com/ar/view.php?ar_id=AR10B00018

Under the NoMachine EULA it limits their copyright to the following programs:
4.1 Commercial NoMachine Software

NoMachine Commercial Software consists of the following computer programs
that have been developed by NoMachine:

 - nxserver
 - nxnode
 - nxmanager
 - nxclient
 - nxapplet
 - nxstat

NoMachine owns the copyrights and intellectual property in and to each
item of NoMachine Commercial Software.
The EULA also names all of the Open Source NoMachine programs separately that remain open.

The URL that Erik provided says:

Note also that to maintain the code ununcumbered of copyright, patents and licensing issues, it is required that you agree on giving to NoMachine a Joint Copyright Assignment, granting the right to use your contributions in future NoMachine software, being this future software open or closed source and intended for commercial or non commercial use. Such a JCA is standard in the industry, being required, for example, to contribute to Apache or to some projects sponsored by the GNU FSF. While NoMachine releases its OSS software under the GPL license (a non-whitdrawable deal, ensuring that the software you contributed will be freely available forever under the terms of this same license), it is a requisite that NoMachine will maintain the full copyright on the software it develops.

So any code anyone's contributed since 2004 to NoMachine's programs (see the above list) just gives NoMachine JOINT copyright with the contributors.

There's only a question if any of x2go's current modules were directly derived from NoMachine's above "programs":

libssh
nxcomp
nxcompext
xcompshad
nxproxy
x2goagent
x2goclient
x2gognomebindings
x2goplugin
x2goserver

Brian



 Article:  #AR10B00018
Added on: 2004-10-04
Last Modified: 2011-01-12
Applies to: NX Licensing
Under what kind of license is NX distributed?
NX Server and NX Client are commercial products, distributed under a closed source licence but all the core libraries (for example libraries implementing X protocol compression), the modified X11 transport libraries (which implement proxying of any standard X client) and the NX agent (the program that runs X sessions on behalf of the user) are released under the GNU General Public Licence. They represent a huge part of the project, currently nearly 80% of all the time and resources dedicated to NX development. See our EULA for specific details: http://www.nomachine.com/licensing

------------------------------

Message: 3
Date: Wed, 2 Feb 2011 09:50:13 +0100
From: Erik Auerswald <auerswald@fg-networking.de>
To: x2go-dev@lists.berlios.de
Subject: Re: [X2go-dev] [Pkg-x2go-devel] Getting things started with
       x2goclient
Message-ID: <20110202085013.GB24710@fgnfs.fg-networking.de>
Content-Type: text/plain; charset=iso-8859-1

Hi,

On Tue, Feb 01, 2011 at 01:53:27PM -0500, Gerry Reno wrote:
> On 02/01/2011 01:44 PM, Moritz Str?be wrote:
> > Am 01.02.2011 18:28, Gerry Reno schrieb:
> >
> >> Unless NoMachine completely rewrote the NX libs from scratch they cannot
> >> change the license nor can they fail to provide any updates made to that
> >> code base.  Once the GPL license is on a code base ALL derivatives that
> >> are distributed in any manner are covered under the GPL no matter who
> >> makes them.

Any code released under the GPL remains this way. This does not affect new
releases done by the copyright holder.

> > No, this only applies if you license the code. Therefore they can
> > re-license  the code under whatever license they wish, as long as they
> > don't violate the rights (licenses) of others.
>
> Without knowing the entire history of the NX codebase, I would say that
> if they were accepting contributions from any contributors that made
> their contributions under the GPL then they, NoMachine, cannot just
> summarily change the license of the codebase.

They have the copyright on all code in NX, see their contribution
guidelines:
http://www.nomachine.com/ar/view.php?ar_id=AR12B00113

To spell it out: Nomachine can relicense the NX code any which way they
want.

BTW this was the first google result for the query "contribute nomachine
nx", a few seconds research would have saved many emails...

Erik