Can I connect +5V GND in reverse to get -5V? Why not?
Of course you can! If you have an isolated part of circuit which needs GND and -5V, just connect its GND to your +5V and its -5V to your GND, and you'll be fine.

However, if you need +5V somewhere in your circuit and -5V somewhere else within the same circuit, you effectively require 10V between these two points. That's why a single 5V supply will not be enough in a general case.