I'm using wxWidgets for my project. It's nice, but the more I use it the more little things about it annoy me. I've looked at Qt and that seems much better overall (but I haven't used it extensively so couldn't say if it's really better). GTK probably isn't a good option as it's pretty linux-centric. Windows users of your program will have to install the GTK runtimes, and your program won't use windows native controls in some cases.
As for SFML (RenderWindow) integration, I've only tried it with wxWidgets. It works in windows but not linux (gtk) or osx.
Overall I'd probably recommend Qt.