GV and Doug Nash/US Gear are 2 very different units. GV is still in production, DNE2/USG isn't. Used units are roughly similar in price from what I've seen - $1k+/-
Very different design. GV is a hydraulically shifted planetary unit similar to an automatic trans in operation and design. It is overdrive & direct only and uses a cone clutch that is spring loaded to the direct drive position. For OD the unit uses hydraulic pressure from an output shaft driven pump to overpower spring pressure and hold the clutch in the OD position. There is also a one way sprag type clutch which prevents the input from rotating slower than the output in forward - this prevents slip while shifting under load and will take load if the clutch slips some under heavy load. Notably, this only happens in forward - in reverse or engine braking all torque is taken through the clutch.
The GV unit is essentially a Laycock overdrive box which goes back to 60's sports car. It is intended to provide a top end overdrive gear - high speed and light load. Yes, you can split shifts, and can tow with it, but the fundamental design isn't really meant for that. The minimum speed is because of the hydraulic pump being output driven - lower speed means lower volume and at some point loss of pressure which means more slippage.
GV is a good unit for it's designed use - high speed highway gears. Same idea as a selectable OD on a E4OD or similar - not really good to tow or run heavy.
USG/DNE2 is a 4 countershaft spur gear (helical) box with a positive mechanical shift system. It can be in either gear at any speed any load forward or reverse. Shifting is a bit harder, but IMHO it's a much stronger heavier duty box. Yes they are long out of production, but they're simple and easy to fix as long as the hard parts aren't trashed. Primary wear points seem to be the shift motor and bearings which are both easy and cheap. DNE2 designs use brass countershaft thrust washers that may be harder to get; USG designs use off the shelf needle bearings. I have also seen wear on the shift collar and fork; I strongly suspect this is an off the shelf GM transmission part but have not determined exactly what it's from yet. The big GV wear part in contrast is the cone clutch which I think costs several hundred dollars if you an even get one and is only available from GV.
Electronics and controls are easy for the DNE/USG. Not sure on the GV, I think you can just run a simple switch and wires as long as you make sure to manually downshift before slowing down too much.
To me the GV is an overpriced box that adds a pushbutton 5th gear and is marginally reliable. USG/DNE2 gives you a heavy duty fail safe splitter box that is much more useful for towing.
