Brienne from ASoIaF carries herself as a man, looks like man, wants to do masculine things (like be a knight). If she lived in Real World year 2020 she could easily identify as a man and talk about how we need to "destroy the cis-hetronormative patriarchy" and think everyone would benefit from reading some Judith Butler. But she doesnt do that because she's in Westeros which is a reflection of Europe year 1200 or so (sprinkled with magic). I cannot speak for the LGBT crowd but is that not "represenation"? GRRM has created a world where the norm is what we're used to but so many of the stories focuses on the outliers. The bastards, cripples and broken things (as Tyrion puts it).
Dunno really. I never thought of Brienne as anything but Brienne: she never struck me as any sort of LGBT statement. It's 25+ years since I read any of ASoIaF and I can't even remember if she was in the bits I read, but in the TV depiction I never got a hint of anything like that about her: I mean yeah, kinda the logical conclusion of what a tomboy is maybe, but that's all I took from it.
