My wife used to keep score on the "big hill" that I have to mow (and spray occasionally). The Gravely rider had good traction for 2WD but could not make it up the last 20' at crest of hill reliably. I came in one time with tires tracks down my back... yes, it happens fast and generally nothing you can do about it (Gravely was near 1000lbs and it certainly left its mark when it went over you. I knew it would be a challenge when I had the sickle mower on the front and going down the hill to start mowing, it pretty much was a semi controlled skid. I cut the field and then tried to get back up. I ended up dropping the mower at the bottom and just bringing the tractor home to get help.
Kubota handles hill fine in 4WD, Gravely machines are just collectors now..