As much as I like a .45, the GAP may have been the worst bullet ever.... Especially since you cannot use it in any other firearm.
Not to thread-hijack, but .45 GAP is probably one of the best cartridge developments in .45 calibre since the 1911. You obviously have never owned one, much less shot one. It was unfortunately proprietary to Glock, but that is how the gun business operates sometimes. The G37 puts .45 ACP ballistics in a much better handling pistol than the G21, and a much lighter pistol than the 1911. The accuracy of the G37 is outstanding, reliability 100% and as fighting tool it is an excellent choice.
The .45 GAP, while still around, was unfortunately subject to the same issues as the .30 SC: ammunition price/availability, existing prejudices amongst shooters, and limited firearms adoption. The .30 SC had the added problems of poorly planned timing and marketing.
I think the .30 Super Carry is a very useful cartridge, and I've had two of these guns since they first came out. It has less recoil and slightly more capacity than 9mm, and reasonable power for a defensive cartridge. Reloading for it is easy. I'm also a .32 nut, so it also appeals to me on a subjective level. But having said all that, I think the cartridge is going to be dead sooner than later. It failed to gain traction initially for a number of reasons, and that window is long closed.
1.
Ammuniton Crisis. It was introduced in the middle of the worst ammunition crisis in our lifetimes, when shooters, hunters and gunsumers could not obtain existing calibres. This did nothing to endear itself to the market, and highlighted the of arrogance of the Vista Outdoors domestic ammunition monopoly.
2.
Limited Firearm Selection. It lacked wide-spread industry acceptance, as only one manufacturer chambered guns for it (Niche Nighthawk does not count). If you did not care for MIM-laded S&W plasticated autos, you weren't looking twice at .30 Super Carry.
3.
Pricing. For a new cartridge trying to gain traction, the ammunition was grossly overpriced. Even as 9mm ammunition became available and the prices dropped (largely due to imports btw), .30 Super Carry range ammo was considerably more expensive than 9mm. When you are looking at two calibres at the sales counter - in otherwise identical guns - and the ammunition for one is 50% more than the price of the other, nearly every "gunsumer" (who is the majority buyer) is picking the cheaper ammo.
4.
Cranky Old Farts. Curmudgeons - of all ages - that don't care for anything that upsets their personal world-view, pooped all over .30 Super Carry as "not doing anything my (Insert favorite cartridge/gun) already does". Well guess what, that is true of the majority of cartridges of all types, as there is massive overlap in practical capability. If you were a dictator and solely interested in Soviet-style utility, you could eliminate 90-95% of all cartridges on the market. While that isn't the point of either the shooting industry or the shooting community, the Cranky Old Fart argument did influence a number of people to not even consider the .30 Super Carry. Just like .45 GAP.
5.
Marketing Debacle. Finally... the marketing was terrible. Incredibly bad timing, and even the YouTube shills they had pushing it looked pained to be involved with it. It was oversold with hype, which didn't jive with the actual performance or package.
If Vista had 1) waited a year or two until the ammo crisis eased and the gun-buying market cooled down somewhat, 2) lined up more manufacturing partners, and 3) put the ammo out at attractive prices, it would have gotten more traction.