My family has had 5 GP. Four of them were great! The one turned on my brother, but we should have known better than to get him in the first place because his dad was nasty as well. Lesson learned.
Our female, Ruthy, that was still have (9 years old) would go with us kids anytime we took a horse out of the pasture. We rode 10, 12, 14 miles. One time we rode 24 miles (mostly at a high speed trot, canter, of gallop... our horses were in shape, and we gave them a half hour rest with water at the 12 mile point), that time the dog came along but she only made it 12 miles and then we went and got her with the truck later (she was in a friend's yard). I should mention that Ruthy got clipped by a car (they are stupid about chasing cars) when she was 3 and broke her pelvis. She just last year stopped going on rides with me, but still goes for 4 mile walks with mom (who walks quite fast). So what I am trying to say, don't discount the GPs for going on rides. The four of ours were/are wonderful with the kids (youngest now 3 1/2), and protective, but not overly so. We only saw Ruthy act aggressive once, and she never growled just jumped in between the person and Sara (2 at the time).
As someone else mentioned, they aren't the best for obeying voice commands (though, we never did train ours, except, no jumping and no chasing chickens). I wouldn't recommend many of the other big guard breeds that are common for sheep guarding, the ones I have met and what I have heard is that they aren't all that good with people.
That's my 5 cents.
One is a bad penny, but the rest were jewels. (so far we lost two to cars, one to trappers (we assume, either that, he was stolen, or the wolves got him), one to 'lead poisoning' and still have the old girl.
If you are planning to get one, look for for one with a broader head, those we found were less skittish. And make sure they were handled from birth.