You might consider the Arkel bag. From looking at images on crazyguyonabike I conclude it's quite popular. In my experience the mounting/unmounting is not particularly easy for the first while - perhaps some silicone grease would help. After a dozen on/off cycles it gets easier, to the point where it's quite easily on/off.
The mount puts it forward of the bars by something like 4-5 cm. It has a map holder, and you could mount the gps either on the bars or on the stem, if your gps mount is the kind that can go either orientation.
It's not perfectly waterproof, although the design is such that the contents stay pretty much dry while the outside may get wet. And there's an available rain cover. On a single bike I would recommend the small size, as the large (which we have on our tandem) is quite large.
More info at http://www.arkel-od.com/us/all-categories/handlebar-bag.html