As I recall, the 45 GAP case is about the same length as a 9mm or 40 S&W case. As you indicated, I think the only problem will be the taper crimp, and that will depend on the particular taper crimp die.
I have purchased several of the C-H Tool and Die 45 ACP taper crimp dies for machines I have reconditioned. Although the quality is very good, I think they are either shorter, or the working part of the die is futher up in the die. As a result, I've had to screw them down so that the top of the die is flush with the top of the tool head. That's fine for 45 ACP, but might not give you enough to work with for 45 GAP. I have an original Star taper crimp die in the machine I use for Bullseye loads, and that die still has quite a few threads showing above the top of the tool head. I think that die could be adjusted down enough to handle 45 GAP
