Hearding dogs aren't hunting dogs, yes they may be smart as dogs go and can be taught many things but those they will naturally excel at are those which are a product of hundreds of years of instinct and selective breeding, that is why they are good heard dogs watch dogs, protectors, that is what their purpose has always been, much like the hunting dog breeds excel at their instinctual types of hunting because of their breeding and instinct, one will not do everything the other is meant for as easily or as well.
As far as rabbit hunting, my dog will chase pretty much any scent or animal it comes across if I let her though she is not a hound so probably not going to excel at rabbit hunting, they are mostly bird dogs with a strong prey drive and many lines are inclined to hunt some fur bearing animals as well though I believe this ability to be far stronger in the actual German dogs, here in the USA most breeders are concentrated on bird and retrieving abilities with the possibility of a fur bearing hunt as a added bonus if the particular dog goes for it, they are considered a versatile hunting dog. We had beagles for rabbit hunting and that would be my dog of choice for that, GWP will excell at bird and retrieve and will chase others but not in the same manner as a trail hound can. Yes the 40-60 lbs is a good estimate some males could be larger, my female was 50 at her last vet apointment probably closer to 60 now as she needs some more exercise.