The term refers to a gown which gaps open in front over a stomacher. The opposite is a Round Gown which closes down the centre front, usually with hooks and eyes. Sack-back gowns are almost exclusively open robes, while gowns with backs pleated en fourreau (i.e. close to the back) can be either type. The reason for this is that the open robe is an earlier style which gradually went out of fashion from about the 1760s on, being replaced by round gowns. Fourreau-style gowns were originally worn on the British isles (and, to a smaller extent, in the Netherlands), and only became fashionable on the Continent from about 1770 on.
Fashionable French ladies turned away from sack-backed open gowns and embraced English fourreau gowns which, by that time, had newly developed the centre front closure. This is why Continental literature distinguishes mainly between the Robe à la française and the Robe à l'anglaise, while English-language literature emphasizes the open robe/round gown distinction.
The gown we are going to look at here is from England. From the cut and style, I'd place it into the 1760s or 70s. The sleeves with neither cuffs nor ruffles suggest a late date, while the fact that it is an open robe speaks for an earlier date. Someone could have removed sleeve cuffs (very early), but the sides of the skirt are not much longer than the front and back, so it wasn't worn over a wide skirt support (late).
I should mention that tese pages are not so much a how-to as an in-detail analysis of an original garment and the techniques used in making it, which are quite different from modern ones. But since I was making pattern drawings and taking pictures anyway...
Warning #1: This is definitely not for beginners. You should have some experience in historical tailoring, preferably with the sack-back gown or caracos.
Warning #2: Do not try to scale up the pattern I'm giving and expect it to fit. Since I couldn't pick the model apart, I sometimes had to make an informed guess about a seam line. You should use the pattern as a guideline for draping your own.
Warning #3: All techniques describes here are period ones. Feel free to use modern techniques, but I'm not going to explain them. Only an expert would spot the differences, and even then only after a closer look.
As always, the stays and skirt support should already be finished. I suggest a proper panier for early (1730-50) or a bum pad for later (1770-). I haven't tried it yet, but I imagine that the long back tip would dip in between the two pockets, creating the look of two huge... well... Let's just say the pocket hoops would have to be rather small.