The gown is late 14th / early 15th c, depending on the specific region and person. The way I've styled it here, I feel it takes on more of a 14th c. vibe. It's a linen cotehardie / kirtle / gothic fitted gown with buttons all the way down the front and on the sleeves in counterchange. The sleeves pin on.
More picture here: http://www.faerie-queen.com/morgan-le-fey/