If this is your first handgun, I'd go with 9mm, cheaper to shoot, and less recoil. If I was a more experianced handgunner, I'd go with .45, although if this is only for inside the house, you may want to consider ACP instead of GAP. If you're going to carry it, even the g30 is about the same size as a g23, within a few millimeters height, so I'd probably use GAP.
As for shotgun ammo, buckshot is better for stopping power, and will go through less walls than a handgun, and birdshot will go through even less walls, though you risk it not stopping.