Can you fire .45 GAP ammo in a .45 ACP pistol?

No.

Maybe.

In an Autoloader the answer is NO.  The .45 GAP is shorter than the .45 ACP.  Depending on your extractor tension you might be able to get away with it. But it is not recommended.  Use the appropriate ammo.

In a revolver that uses moon clips, yes.  Note that .45 GAP is loaded to .45 ACP +P pressures.

Yes it can be done if loaded in one at a time.  I would not recommend it because of the increased pressure of the .45 GAP.

Only if you can get the extractor to hold the round against the breech face.

The gap cartridge is to short to properly headspace of the front of the chamber.

Not recommended.

This is correct; .45 GAP should not be fired in a pistol although as stated above by someone else, it can be fired in a .45 ACP revolver that uses moon clips.

I managed to occidentally fire several rounds of 45 GAP in a 1911 and when the 3rd round blew it's primer I had the opportunity to look into this.  The answer is that you cannot do this.  It won't headspace properly and several bad things can happen.   Also, you can't fire single rounds in general in a 1911 unless you load the round into the magazine first.  The extractor is not designed to work any other way.

Actually it is designed to do just that…

That is why the front of the extractor has that bevel on it and relief in the extractor channel to allow it be pushed over by the cartridge rim (also beveled) then snap back.

There are some who think that 1911 parts are made out of glass.

On the contrary, it was designed for Combat conditions to operate even from horseback and under all conditions.

But while the .45 GAP may work sometimes, you are definitely taking the exit to failure and on a very short ramp

I hope you know I mean this with all due respect when I say that you are entirely incorrect on this topic.  If you make your extractor 'snap' over a cartridge head, over a very brief time it will be out of adjustment and you will start having malfunctions centered on this critical part.  In some cases and if a habit is made of doing this,  it will simply break.  The extractor is designed to have a round slip under the hook from the magazine and that is the only way it is designed to work.  You may be able to force it to snap over a cartridge, but you are breaking the gun.  And a 45 GAP will not headspace in a 1911 pistol meaning you can't fire it under any circumstance.  I am no expert, but I am certain on these points.

I never kept track of the number of rounds but I do this from time to time, drop a single round in and release the slide,

My oldest 1911 in a Gold Cup I bought in 1975 – I’ll let you know when it detunes or the extractor snaps off

I have fired a couple of magazines of 40 S&W through my 10mm Delta Gold Cup. They are about 1/8” shorter than the 10mm. but worked without a hitch. (now these were not dropped in loose)

No I do not recommend 40’s in a 10mm or GAPs in a ACP!!! Yes even though it “worked” for a few people, problems like you saw are the more likely outcome.

Edit: I should add – if you want to believe in the ’glass gun’ not a problem, just an area we will not agree on…

We are agreed that GAP in an APC is not a good idea!

Thanks guys, that clears it up!!  I was just curious about this round, what is its purpose, is it faster than ACP, less recoil, better performance, i.e. why the shorter round, for compact pistols?

Yes, Glock created the round so they could have a .45 chambering in their 9mm and .40 sized pistols.  Do NOT attempt to fire it in a pistol chambered for .45 ACP.