This is based off the last time I took a GV out, which was in AH2. It may or may not have any use in AH3.
Don't try to go full throttle up a hill. That is when it'll up/down shift constantly. Just "select" a speed by holding the W keys and keeping an eye on your "hud" speed gauge. Once you start up a hill, you'll see your vehicle reach a certain speed before she upshifts to the next gear, then very shortly after down shift. I usually go 2-3MPH slower than up-shift point and find myself climbing a hill at a nice, consistent speed.
As for going down a hill...I tend to tumble down it.
