Impossible. The Gamemaster items are 100% unique.

While it's true the GM robes are unique, it's not true that you can't find things similar. In fact, Alliance can get a pretty darned good knockoff, though when female characters wear it the graphic is slightly different. It's called Fur Covered Robe and it's an easily aquired quest item. Females can pair it with a Brown Linen Shirt and you're looking pretty Game Master-y.

For the hood, I recommend Augural Shroud, although the NPC that drops it seems to no longer be in game. But the color matches perfectly, and it's BOE, so if you get lucky and find one on the AH scoop it up.

And if someone decides that you're wearing that outfit to impersonate a Game Master, prepare to have the Fury of Martin fall on you from a great height.

If you're trying to trick people into thinking you're one, sure; but I don't see the harm in wearing it around for a laugh. Same as you can wear fake cop uniforms on Halloween or to a party, but if you start trying to pretend you really are one, that's a paddlin'.

As I said (or at least tried to imply), it doesn't really matter what your intent was - it only takes one to file a ticket, and then the best case is that they let you off with a warning. Personally, I wouldn't risk it.