Well, thanks to the information in Portal 2, my answer is...
...So, yeah, "nothing" is my answer. But what I wouldn't do to have a GLaDOS of my own...
*daydreams of running through a field towards GLaDOS, only to be shot with a rocket*
Nothing, since the gun can only shoot at surfaces covered in moon rock goop, making it useless everywhere outside an experiment chamber. If you gave me a gun that shot moon rock goop AND the portal gun, I'd still say nothing, since Mr. Johnson proved being around concentrated moon rock goop for a long time causes cancer.
...So, yeah, "nothing" is my answer. But what I wouldn't do to have a GLaDOS of my own...
*daydreams of running through a field towards GLaDOS, only to be shot with a rocket*