[Edit, 1/14/2013: Newcomers, you may want to see my tag for the turque, which will show you all of the Galerie des Modes turque plates I've translated so far. They're the same plates as below, but they have longer descriptions that I've also translated, that give the conclusions I took ages to get to here.]
Gallerie des Modes, 1780
The description for this trio is interesting. It reads: "Robe à la Turque ou espèce de Circassienne, mais différante des autres; elle a un collet comme une robe en Lévite, et une très grande écharpe blanche nouée à la ceinture; le juppon coupé; aucune garniture. Cette robe dont nous donnerons le développement de profil et par derriere, attira tous les yeux du Public, lorsquelle parut pour la premiere fois au Palais Royal, au mois de juillet dernier 1779." This translates to (and please, please correct me if you think I'm wrong): Robe à la Turque or a type of circassienne, but differing from those: it has a collar like a Lévite, and a very large white scarf tied at the waist; the petticoat is cut(?); no trim. This gown, for which we will give the development(?) from the profile and from behind, attracted the eyes of the Public, as it appeared for the first time at the Royal Palace, last July, 1779.
Amiable Constance holding a dog's leash and dreaming of what her heart loves: her dress is à la Turque and her hat is à la Mongolfier, [???] a bather, and trimmed in with a ribbon attached to a buckle in the English style with a feather.
The young Eglea crying for the absence of her lover: she is dressed in a robe à la Turque with gauze sleeves which pull up with ribbons, she is coiffed with poppy-colored ribbon in her hair.
I have seen a reference to the turque being a variation on the polonaise (back to this in a bit). The Short Glossary I referenced in the circassienne post says: "tight bodiced gown with attached open-front outer robe, closed in front, with draped sash and funnel shape sleeves."
The back on this gown certainly resembles the polonaise. It's difficult for me to say whether or not there is a waist seam, but my instincts say no, mainly because of that front view and the back's resemblance to the polonaise. Because of this, it's certainly possible to describe the turque as being a polonaise variation ... but I disagree with it in the instance that I found that description, as the blogger was calling a française worn rétroussée dans les poches a polonaise, which gives a completely different impression. The main differences between the two are in the sleeves and the skirt: the skirt is cut straight, like that of the anglaise, and the sleeves are flared mancherons over wrist-length ones.
I think this is one gown with a petticoat - that the underbodice is sewn to the overrobe and the undersleeves are set in, rather than the turque being a separate robe worn over an anglaise, mainly because it seems easier and no underdress is mentioned in the descriptions.
Like the circassienne, the "oriental" details are key. The drooping, asymmetrical scarf and the single tassel would have been seen as exotic elements of dress, and the scarf may have been specific to the turque. As of right now, I'm not sure what the Lévite attributes of the collar are - possibly the ruffle, or the squareness, or that swag in the back - I'll come back to it when I study the Lévite.