Gui: Qt vs. GTK vs. ?

Hello!

I need to make a GUI for my application and I am not sure which toolkit I should use therefor.
My requirements:
- must run under windows and should run under linux
- making widgets should be easy (I need to make non-standard widgets)
- there should be a community or at least resources where I can read/ask specific questions.
- freeware (we have Qt4 in the office)

What would you recommend? GTK or Qt4? Or are there better things?
The GUI itself consists only of a main (dialog) window where the widgets are placed on.

Greetings,

Have you looked into wxWidgets?

yes, I have looked at it and forgot to mention it. Probably this one is the best choice.

Yes, wxWidgets is the best choice since it's easy to use and the look of your application will look native to the OS that's running on.