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Yes, JD and Incama are right. In fact, XOOPS is Free Software, but free as in freedom, not gratis. So you can sell the system code as well. No-one will stop you from doing that.
But, you cannot restrict what your clients do with the code, nor can they restrict others if they want to distribute it. The GNU GPL License model is all about the freedom to use the code as you see fit, and granting everyone else this same right. So, use without restrictions, for yourself and anyone else.
One thing you should take note of tho is combinations with non-GPL licensed code, that can be an issue. If this applies to what you're going to do, contact me via PM or e-mail, and we'll discuss the consequences.
Herko
