The 'read me' file included with NextGEN Gallery states the following:
"Copyright 2007-2012 by Alex Rabe & NextGEN DEV-Team
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version."
Essentially, this means that you have the right to redistribute NextGEN Gallery but you aren't allowed to charge for it. You can use it and install it on your client's websites and charge them for your services but you CAN'T charge them for the program.
This is my understanding of the GNU GPL licensing. Do not take my advice as legal advice. It is only my opinion. Please check the GNU GPL website for more information.