In 2.6 the minimum amount of windows is 2, in the (currently available) 2.7 or the soon-to-come 2.8 you can have a single window mode.
Having 2 windows isn't that much of a problem in my opinion ... My Gimp is set up like that, one window for the image and one for the toolbox, layers and tool options. Simple but enough for everything.
Gimp-2.7.3 is actually pretty stable if you ask me. http://www.partha.com It comes in portable version if you prefer. Single window mode is just a little buggy yet though.It will not save single window mode on exit. But it's no big deal to click on button in Windows/single window mode. Screen shot -