Then it's not a GPS coordinate, it's a lat/long one. GPS measurements include altitude; whether or not one chooses to use it is a different matter.

Bingo. In my apartment building none of the units had numbers. I could point you to my unit, which was one of two doors on my floor, the other leading to a unit directly above me. So even GPS + Floor wouldn't quite me enough, although that's close enough that we could figure out who the package was for.

Then the full name should be anough to determine the floor. More problematic though is the inaccuracy of GPS. Where I live, my smartphone's GPS encircles 3 buildings.