As CoolCat mentions, if you sell GPL software, you still need to include the GPL license and access to the source code to your customers - that may include the derivatives such as plugins, etc. though you would still be the copyright holder depending on how much modification you did.
Here is some excerpts from the GPL FAQ and a document on selling GPL software:
Does the GPL allow me to sell copies of the program for money?
Yes, the GPL allows everyone to do this. The right to sell copies is part of the definition of free software. Except in one special situation, there is no limit on what price you can charge. (The one exception is the required written offer to provide source code that must accompany binary-only release.)
Can I release a modified version of a GPL-covered program in binary form only?
No. The whole point of the GPL is that all modified versions must be free software—which means, in particular, that the source code of the modified version is available to the users.
The FAQ provides examples of distribution, selling, etc.
This document also talks about selling: http://www.gnu.org/philosophy/selling.html
But probably a good idea to talk to a business lawyer and/or the GNU/FSF groups for more details on how to proceed if you chose to sell GPL software just to make sure.