GMDesk is a little Adobe Air application that lets you run your Google apps in a stand-alone browser. This could be handy for some folks, particularly those who frequently close their browser and would lose contact with Google Mail or Calendar or Docs this way: it lets you treat Google as a separate matter conceptually, in effect. As you'd imagine, there's a menu (and easy shortcuts) that let you switch between the various Google applications you use.
Given that browsers load so quickly now (I'm assuming that IE loads fast), it's not so clear that there's a need for a separate browser dedicated to Google. However, it may appeal to the way some people work, which, after all, is the point of all of this.
Mac users are likely to be less interested, perhaps, because of Fluid, a program that lets you create dedicated browsers for whatever purpose you wish. As well, Adobe Air applications have the annoying feature in OS X of closing down when the open window is closed, unlike all real Mac apps.