This is due to the fact that the team working on NZP has still failed to release the source code of the project which is required by GPL. This is outright copyright violation to take someone's code, in this case the Quake engine and use it against the license that id software has set for it, which requires source of the deriative works to be released.
I tried to reach out to NZP team and requested the source, but their answer was locking the thread and telling their engine is 100% own and custom (http://nzportable.com/forum/viewtopic.php?f=5&t=1295(https://www.dropbox.com/s/xrm3kesh3jkkb ... _scrot.png))
The whole project is indeed based on Quake and thus violates GPL terms which one can familiarise with at http://www.gnu.org/licenses/gpl.html
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