The reason why the military primarily used .45 for such a long time is because the Geneva Convention disallows conventional military forces from using HP ammunition. Anything FMJ just makes a hole in the target with minimal actual round expansion. Therefore the logic behind it is the bigger the FMJ round the bigger the hole. The bigger the hole the more blood loss and impact on organs. The hydostatic shock is also going to be different, especially with older ammunition technology. There are also things like tumble in bullets, but that's for another time. In this current day with the level of technology we have with ammunition there is no real reason to go beyond 9x19 as a pistol round to effectively kill a human being on a private, or civilian level of choice. However it is also fun to shoot .45 and will still kill you just as well as a 9x19 round.
Also - the .45 GAP does nothing better than anything else other than make the round more compact and expensive, which is why no one really uses it except certain governmental agencies.
That being said, they obviously want to make things as diverse as possible by adding the .45 ammo. It's going to be nice to see a Glock in the game either way.