Glock, glock!
Gwho's There!
GAP .45
GAP .45 Who?
Exactly.
I think you're confusing cause and effect. The reason why the ammo is uncommon is because the pistols failed to attract buyers. Any target market for .45 GAP disappeared after short-frame .45 ACP models were introduced.
As I see it, people who want "more bullet" than 9mm or .40 mostly fall into a couple of categories. The first are .45 ACP/1911 users/fans, and I don't mean that as a pejorative. They simply weren't and aren't interested in a new round. Then there's the "fortay is crippled" crowd who embraced 10mm. Had .45 Gap been a, say, 28,000 psi round, they might have been interested, but then Glock couldn't use the same frame.
As we saw with .30 Stupor Carry, unlike rifles, it seems next to impossible to establish a new mass-market pistol caliber. With modern self-defense ammo, 9mm, .45 ACP, and, yes, .40 satisfy 95+% of the market.