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Yeah... what he said.
Seriously (This is a serious matter), you can not restrict the redistribuion of the work. Even requiring a tag line at the bottom of the module output is against the GPL.
The GPL is also very clear bout your rights regarding software that is based on GPL work but illegally redistributed under a more restrictive license. The parties who recieve the derivitive work still have a GPL license to it, and the party who imposed the restrictions have no license to the original work or it's derivitives (even though they created the derivitive) They accepted the GPL of the original work and must apply it to the derivitave.
You may be the author of the derivitive work but are the third party of the original! So by definition, your module is a dirivitve work of XOOPS if it calls any code coverd by the GPL. It therefore MUST be GPL as well. No restriction.
JMass
Edit - from:
http://www.gnu.org/licenses/gpl-faq.html#GPLModuleLicenseIf I add a module to a GPL-covered program, do I have to use the GPL as the license for my module?
The GPL says that the whole combined program has to be released under the GPL. So your module has to be available for use under the GPL.
But you can give additional permission for the use of your code. You can, if you wish, release your program under a license which is more lax than the GPL but compatible with the GPL. The license list page gives a partial list of GPL-compatible licenses.
