Except Jaisalmer Fort, Gadisar Lake is often shown as the cover picture of Jaisalmer on some travel blogs/websites. The lake isn't far from Jaisalmer Fort, 15-ish minute walking distance. Easy to orientate, just follow the road downwards, the area you walk to the lake is quite residential, with some odd haveli hotels (lovely traditional styled private mansions).
You can take a boat tour too, which is popular among local tourists. Lake area is quite busy, but I saw some drive to the other side of the lake, where you can get a better view also can take better pictures. But it's not so reachable on foot, as there isn't a walking route along the whole river, otherwise would be better.