GNU GPL allows you to modify code and distribute it as well (paid or unpaid) , as long as you make the sources themselves available (i.e. unmodified coppermine). Don't strip the reference to coppermine from your code, and don't remove/obstruct the "Powered by Coppermine" footer though.
For details on what you're allowed to do and what not, check the GNU GPL at
http://www.gnu.org/copyleft/gpl.html and/or
http://www.opensource.org/licenses/gpl-license.phpThere is often a misconception, where users don't understand that the in the open-source concept, the word "free" in "free software" is meant as in "freedom of speech", not "free beer". There are other companies who make a living out of software released under GPL (e.g. IBM, RedHat etc.), so generally speaking there's no reason why you shouldn't try to do so.
However, watermarking, PayPal integration and most of the other things you mention are available as free mods for Coppermine, so I can see little point in trying to sell your modifications. We're currently working on a plugin API that will enable users to apply all sorts of mods by just enabling them in a plugin manager within the coppermine user interface, making modifications that won't go into the coppermine core code dead easy. The next version of coppermine will even be shipping with some of those plugins, users will just have to enable them with one or two clicks.
Please make sure to refer to coppermine.sf.net if you decide to sell your modifications, but please also make sure that your possible future customers understand that this board is not the place to look for support on hacks/add-ons
you provide. Please make sure that the mods you coded have actually been done by you entirely and have not been ripped from the mods available on this board.
Joachim