KRAIG - You are correct, there is a brief break in power between declutching & braking with a GD. Which can induce a tire to slip on wet/slick grass. Same thing applies going uphill but to a lesser degree since weight transfer is in your favor with the drive wheels on the downhill side.
Best not to try to shift gears up or down when ascending/decending hills, and hope the transmission poppets are in good shape so it doesn't shift itself into neutral. I forget exactly where I read it but on many IH tractors equipped with Torque-Amplifiers, the tractors would "Coast" or freewheel in the low side of any gear. I remember reading about someone getting an IH tractor & loaded wagon up to about 50 mph by accident going down a l-o-n-g hill on IL Rt. 150 between the Quad-Cities & Galesburg, IL. One of the three little planet gears for the reduction unit also didn't like being over-speeded like that and made a hasty exit from the clutch housing of the tractor almost splitting it in two.
A hydro will gain speed going downhill and slow up going uphill, both due to slippage in the hydro and the change of engine RPM as load increases or decreases. A GD will also change for the same reasons except there's no slippage in the hydro, so the change will be much less.
GD's are best for long duration pulling like plowing, constant speed operation like spraying & fertilizing. Hydros are better for mowing, blowing snow, tilling. For general yard work like pulling carts & lawn sweepers, either work fine, but a hydro does allow faster ground speeds at reduced engine RPM which may save gas.
KENDELL - Was it a "dark cloud" or a HUGE UFO like in the TV series "V". Probably a BIG red center with a white ring around it?