The 1968 Gun control act did NOT ban guns by caliber. While it was effectively a ban on a lot of small guns, it was not based on the caliber of the gun, but the size of the gun. Along with the "sporting purposes" clause, the GCA 68 awarded "points" to the gun based on the total of its dimensions. IF a gun didn't have enough points, regardless of caliber, it was not allowed to be imported.
The stated reason was to stop the "flood" of cheap, small pistols, aka Saturday Night Special. Couldn't do much about those made in the USA, but by gosh, we can keep foreign ones out!!...
(if I remember right, there were also rules about the metal used, frame melting point etc.)
The most famous example of this rather stupid regulation was it banned quality guns as well, if they were "too small", such as the Walther PPK.
The PPK, in ANY caliber, didn't score enough points to be imported, essentially because the grip frame was 1/4" too short to make the arbitrary import minimum.
Just another example of a gun control law claiming to do one thing, but also doing other things as well. Walter's response was to create the PPK/s which DID meet the import requirements.
There are no "banned calibers" in the GCA 68.