A license is just that, a license to use the code. The copyright holder of a piece of code (or really any other copyrighted data) may license it to whomever the

A license is just that, a license to use the code. The copyright holder of a piece of code (or really any other copyrighted data) may license it to whomever they want, however they want. The GPL is just a blanket license giving permission to everyone provided they follow the restrictions put forth by the GPL. If you are the sole copyright holder (because you are the only person who worked on a project, or because all others assigned their rights to you) then you are allowed to provide any additional licenses besides the GPL. A lot of open source projects (see Qt, MySQL and a bunch of others) work this way, only accepting patches from the community if the patches come with a copyright assignment. This allows them to charge for closed source use of the library while at the same time allowing the open source community to benefit.