A quick skim and my own personal brain (which is probably wrong in a few cases) says this:
M691 Diana - existed, but was never adopted
M990 ADA - T2K seemed obsessed with giving the US Army AAA capability that it never went forward with!
M917 ADATS - the ADATS system existed, but in US testing was always on a M113 chassis I believe, and never adopted.
M920 Hellfire - pretty sure this is a total fabrication
M757 Blazer - likewise.
M975 Roland - prototype only
ADATS took a little too long for the Army to procure it, but it would have gone on a Bradley chassis. The Canadians, who not only participated in the program but actually procured it, used an M113 chassis since they used that chassis already but had no intention of ever procuring the Bradley.
The M920 is a kind of "super M910 ITV." The M901, a dedicated tank destroyer with the TOW missile system, was used by mechanized infantry battalions. This made sense when the standard infantry carrier was the M113, but when the Bradley came along it also mounted TOW. And the M113 couldn't keep up with the Bradley. So GDW figured the sensible next step would be to put a better antitank missile, the Hellfire, on the Bradley chassis. In reality there were no ground-launched Hellfires until the 2000s, and these were developments of the active radar homing Longbow Hellfire not the semiactive laser homing Hellfire.
The fictitious M757 Blazer is eerily similar to the real-world M6 Linebacker. Both were M2 Bradleys adapted for air defense and both mounted four Stinger missile tubes. The M757 had a three-barrel 30mm chaingun (which I don't think actually exists) whereas the M6 just had the single-barrel 25mm chaingun from the standard Bradley. Upgunning makes a lot of sense here since you don't want your air defense vehicles to be outranged by the enemy attack helicopters.
Another thing to note is that GDW referred to the Sgt. York as the M988 in both Twilight: 2000 products and in wargames such as Assault. The actual designation was M247. I am not sure where the M988 designation originated.
As to why so many GDW invented so many forward air defense vehicles, well, this was a perpetual problem spot for the Cold War-era Army. Since the 1960s, program after program had been canceled and instead the Army got a series of interim solutions. GDW figured that the Army would actually commit to something during the late 1980s or early 1990s. It did not and the Cold War ended with the Army still fielding the obsolete PIVAD and Chaparral systems.