The G Body windshield are held in with a urethane sealer rather than buyle (SP) . They are a huge PIA to cut and you canít get in a hurry on it. Iíve also tried the tool you show on them and itís about worthless on urethane.
The best Iíve found is a 2 ft handle that is made to hold a box cutter type blade, but even then plan on taking an hour or 2 to get the windshield out in one piece.
I know most glass shops use an electric or air cutter to remove the old windshields Iím not sure if they are like these or not.....
but if I had to do another one I would sure at least buy the cheap one and give it a try......of course the down side you need to be around compressed air.
For no more than a new replacement windshield costs itís probably well worth just having a glass shop do it in the first place. To properly replace the glass you will still need to use the urethane which really requires a minimum of a compound action caulking gun (an electric one is actually nicer but usually well over $100)
Plus the urethane itself at about $10 a tube (1 tube would probably be enough but would you really want to find out after you got started that itís not).
So basically to do the job easily and correctly youíre basically looking at a tool and material cost that is right at the cost of having it professionally done. If your budget is too tight to afford the tools to make it easier plan on trading effort sweat and aggrivation for the cost savings.