I have a German Shorthair Pointer that during the winter months despite how much I feed her gets extremely skinny looking just to fatten up with much less food in the summer... Is this weight fluctuation normal?
Yes, it is normal if the dog spends some time outdoors each day. With such an athletic breed I can't imagine that you don't keep it well exercised with plenty of running time. They burn a lot more calories in the cold weather. You may even notice a drastic drop in appetite in warmer weather.