When the over dress is split open from waist to hem in front, it is a robe, rather than an overdress; and you're right, it would have needed to be worn over something!

Judging by the fashion plate descriptions (I know, I natter on about these constantly, but they're about the only period-concurrent literature we have re the fashions worn), these kinds of gauze dresses are worn over satin "slips" or underdresses. I think the use of satin is because it allows the cotton gauze to flow gracefully over its surface. Cotton over cotton would kind of stick and lose that drape-y effect.

Louisa
Mistress of the Garderobe