I never got the "brutally realistic depiction of medieval life" part about Game of Thrones. Yes, they're loosely based on the War of the Roses, but there are plenty of things that aren't authentically medieval: hay bales, all the girls reaching puberty and being married off by their very early teens, the trade and travel that exists between continents, telescopes, the level of medical knowledge, the slave trade and so on. Some of these actually resemble the early modern era (or later) in some ways, in spite of the lack of printing presses and gunpowder.
And the architecture and clothing and so on depicted in the TV show don't seem terribly medieval either.
Here's an article about how AGoT etc. isn't authentically medieval.
But it's fantasy, so who cares if it's perfectly realistic? The only time it becomes annoying is when someone uses the "but it's realistic" notion to justify something some readers criticize in their work (like no interesting women as characters, which to be fair to Martin, he doesn't have a real issue with), or to show life as unrelentingly bleak and people as unrelentingly cruel (even in the real middle ages, I suspect that there were some bright spots and at least a few good deeds that did go unpunished from time to time, and a few friendships and love affairs that worked out).
I suspect that grimdark has been popular lately, not because of it's "realistic" portrayal of the middle ages, but because people are actually pretty cynical and pessimistic about human nature in our own times.