I’ve always been frustrated with the lack of proper full screen preview in Gimp. By default if you enter fullscreen mode (F11), it just gives you a bit more screen real estate but keeps all of the menu bars, rulers and other things that distract you from the image.
I found that you can customize what is displayed in full screen mode, and also on what colour background your image is shown. By turning all things off in full screen mode, and selecting a background colour of choice (I like black) it works well as a full screen preview mode.
Here is what my preferences look like now (click here for a larger version):