George R.R. Martin. But beyond him, I really don't know any who've done that and actually sold well. Paul Kearney does some grittiness, but he's not exactly a bestseller. I had to get used copies of his books off Amazon because they've been out of print for forever.I wonder how grittier writing would sell in this genre. Personally, I like my reading to gloss over those nasty bits like getting sick in the winter (no one ever catches a cold), going to the restroom, menstruation, the horrid smell because no one hardly ever bathes, the reek of sewage clogged streets...you know that stuff.
Personally, even if you don't focus on the nastier aspects of medieval life in a fantasy, I feel that unless you can write a way to negate some of that stuff, then some mention of it adds that much more to your world.