First off I should point out that Gadwall do not always come in neatly opposing pairs like this! But this is a nice female and male duo, with the female on the left looking a lot like a Mallard female. The most notable difference is the white wing bar (present on both male and female) instead of metallic blue/green on a Mallard. However in both species that may be hidden entirely depending on how the wing is positioned, as in the picture above, in which case you're back to much more subtle clues for identifying the female, it being a bit smaller and daintier with a different bill. That said the black stripe down the centre of the yellow bill that my identification guide promises doesn't seem evident in the photo above! Frankly I find that identifying the nearby males to which they're attached is the easiest route, but maybe I've been fooled and Mallard females are in fact hanging out with Gadwall males.

The male is almost entirely grey, with a black rump and white belly. The grey body is actually very finely textured and rather beautiful, as shown in the close-up below. Apologies for the fuzziness of the picture, but Gadwall are much more timid than Mallard and tend to stay a good distance from people, so this is the best my 400mm lens could manage on that day.