Ballistically? Intended to be identical.
Physically? The GAP is shorter. HOWEVER- do not try to shorten .45 ACP brass and make your own GAP brass. The real GAP has a thicker web at the base to prevent blowout- they operate a very high pressure, with little support of the brass. Small pistol primers.
I have always considered it to be an answer in search of a problem. With SOME Glocks, the frame was not big enough for the ACP, so the shorter GAP was created.
And it allowed Gaston to have his name immortalized on a cartridge.