You can make money from something thats gpl'd right?? You just have to make the source your modifications available.
Basically, if you don't modify the source to the GUI then you don't have to release your source code. And if you do change it then you are required to release the changes publically (with the same license).
You can still do commercial software with GPL'd code, just look at Unreal Tournament 2004- it uses SDL (Simple DirectMedia Layer), which uses the GNU LGPL (Lesser General Public License), for the Linux version.
If you really want to know, you should read the license and find out. It's written well enough for most people to understand easily.