GPL license question.

I'm doing a project that I might want to try to sell or do something with later on. For the prototype, (will probably port to something more web friendly) i'm using Java with QT designer for the UI. Now my question is, if I use the open source license for QT, what kind of copyright issues will it cause? Will I only have to share the UI as open source or the whole system? Can potential competitors use my UI to provide the same product with their own system behind it? Any recommendations/java sucks comments?

license info:
http://trolltech.com/products/qt/licenses/licensing/opensource

Now that I think about it, I get that QT is released under GPL and if you want to change QT you have to keep it as GPL, but how the hell are they supposed to limit the applications made with QT? QT is a tool that creates java files from a visual design, just like gcc creates executables from code.

http://en.wikipedia.org/wiki/GNU_General_Public_License#Common_misconceptions
says: "For example, using gcc [GPL] to compile proprietary software is allowed."