I wasn't planning on giving my opinion, but thinking things over if given the choice I would go with the GAP. I'm already set up to reload it (anyone who loads .45 ACP is) where the 9x18 requires me to get a new die set and bullets that would only be used in the 9x18. I already have plated 200gr .45 bullets, I use them in .45 Schofield.
There's also the benefit that the GAP is a .45 that only uses small pistol primers, which means I don't have to sort thru brass when I want to shoot some .45 with small primers, which is what I have the most of, it's less time sorting.
The Glock itself is a proven platform, lots of spare parts available. Trigger is typical and something I know well, the Mak trigger... idk how good or bad they are.
Probably the best aspect of the 9x18 is the ammo will be cheap, cheaper than GAP, but availability for both is questionable, so I would still rely on reloading to ensure I can shoot it and that all favors the .45 GAP.
Capacity is equal between the two pistols and I'm willing to bet the price of a used GAP Glock is less than that of a Mak and has probably been shoot less too.
The Mak may be steel framed and have a DA/SA trigger (I like DA/SA) but there are IMO more drawbacks to the pistol and caliber than with .45 GAP in a Glock.