Brandon Sanderson? Grimdark? I don't think so.
Sanderson is sometimes seen as being on the edge of grimdark, like Lynch, but as a prominent author who uses a lot of grimdark elements and themes, and with his starting with Mistborn, he's often presented as part of the movement. I'm sure the academics will have lots of nice discussions about it down the road.
I do think the modern grimdark period was led by Martin's ASoIF,
Nope, he's just their grandfather ancestor. I'm sure a lot of the grimdark authors have been inspired by Martin's Song of Ice and Fire, but they've also been inspired by Howard's Conan, Michael Moorcock's Elric, King's Dark Tower, D&D and Warhammer games, horror and dark fantasy writers like Clive Barker, and writers like China Mieville, Matt Stover, Stephen Donaldson, and Guy Gavriel Kay. Dark and nihilistic fantasy have been around a long time and have been highly successful. Martin did not lead a reactionary movement from Tolkien -- his Song series came about after thirty years of fantasy writing from Lord of the Rings. Let's not make dozens of major, prominent authors of the seventies-nineties disappear just because it sounds good.
Likewise, Martin was not called grimdark in 1996 when Games came out. (Bear in mind that he was better known as a science fiction writer than a fantasy one in 1996, though he'd written fantasy too.) No one in written fantasy was called grimdark. Instead, the term was used in games and came from the tagline and marketing for the game Warhammer 40,000 back in the late 1980's. It started being used in the oughts about various sec world fantasy writers like Abercrombie, Bakker and Weeks. If things had gone differently with the New Weird genre movement in the late 1990's to 2005, they might have just been folded into that, but that was also of its time, which had passed, and the grimdark authors were doing something slightly different, so people came up with a new term for it. It's not a new type of fiction that no one had ever seen versions of before, however. They never are.
But the particular themes, style, and emphasis on particular content -- mainly horror monsters, very graphic violent battle action, political dystopia, and nihilistic failures of redemption in anti-hero and villain main characters -- those were identifiable as a core that several authors who were getting a good reception were using to one degree or another around the same time. Other terms that were floated around for it were nihilistic fantasy, gritty fantasy and dystopian fantasy, but grimdark was the one that stuck, which is usually what happens.
Women write grimdark fantasy and were around in the time period. But there is a cultural expectation unfortunately, a myth if you will that still has social weight, that women are usually inherently inferior at writing violent battle action, and dark, gritty amoral horror. (Never mind that women have always been a big part of dark fantasy and some authors like Elizabeth Moon have been in the military.) Women authors in general have a very hard time attracting SFFH media attention if they are writing secondary world fantasy fiction, which is why a lot of them went to contemporary fantasy in the oughts. And so women authors who might be considered part of grimdark were not necessarily
called grimdark, were not discussed and presented as grimdark authors. Instead they might be called dark fantasy, gritty fantasy, horror, etc. But sometimes, some women authors like Karen Miller and Mazarkis Williams have been mentioned as grimdark and it might happen more often as things evolve, as happened to Jo. (Remember, grimdark is now a stylistic sub-sub-category in the sub-category of secondary world fantasy. It will be thrown around as a term a good bit, and also sometimes outside of secondary world fantasy as we've seen.) Authors like Robin Hobb, J.V. Jones and C.S. Friedman are also seen as ancestors/inspirers for grimdark writing, as well as books like Mary Gentle's Grunts.
A sub-genre is another word for sub-category -- it means a bookselling category that also works as a common reference term. A subgenre/category is secondary world fantasy of the category fantasy. A sub-sub-genre or category is a smaller set of general content, that is part of a sub-genre/category, like fantasy-of-manners in historical fantasy/secondary world 1800's like fantasy. Fantasy likes to group its subgenre categories by setting mostly, including atmosphere/tone of setting. So it has secondary world (a non-Earth world,) contemporary/urban fantasy (contemporary Earth or alt Earth,) historical/alt history fantasy (historical or alt historical Earth,) futuristic fantasy (in which the fantasy occurs in a future time of Earth, including in space and post-apocalyptic scenarios,) portal/multiverse fantasy (some sort of transport between worlds including often Earth in different dimensions,) fantasy horror (horror that uses fantasy elements for its horror,) dark fantasy (fantasy that has a dark, Gothic, dread-filled, horror infused atmosphere, themes and setting,) and comic/satiric fantasy (fantasy with a satiric humorous atmosphere ranging from broad farce to dark tragicomic satire.) The last four obviously could take place in a historical, contemporary, futuristic or non-Earth setting -- they cross since portal/multiverse has multiple settings and the others concentrate on the atmosphere side of place. (Science fiction organizes its sub-genres by type of science/tech in the story, which can be setting related.)
There is also YA fantasy which is a sub-subgenre/category of the category of YA fiction, in a different market -- the YA/children's market. YA fantasy doesn't usually break itself up much into sub-categories and gets referred to just as YA fantasy a lot of the time. However, with YA so large and more media attention given to it, that's been changing. All the types of fantasy fiction we see in the adult market for fantasy fiction, you'll find in YA too, and of course YA and adult fantasy titles often get talked about together.
We also use the term genre to refer to specific genre movements that appear to crop up at different times. Sometimes these genre movements then also operate as sub-sub-genres in bookselling and fandom: cyberpunk, steampunk, magic realism, fantasy-of-manners, Weird fiction, New Weird, and it can become pretty endless as people like to play around with descriptions. But the descriptions are meant to serve as tools of reference and discussion. Genre movements may stay mainly in one type of setting, or they may cross as an atmosphere.
Grimdark is basically a sub-subgenre of dark fantasy that was mainly limited to secondary world settings, and again, usually pre-industrial ones. Essentially, the image of the grizzled barbarian beserker warrior, sitting on a rock in the rain leaning on his battle axe handle while a monstrous shadow looms towards him in the background and a soldier's split corpse lies at his feet -- that's the logo of grimdark. It's a very old logo -- they didn't invent it, but they were trying out particular themes and styles with it. How dark and grim each author got varies, but there's enough commonality between them that people grouped them together as conceptually and stylistically related. And then we contrast and compare other authors, past and present, to that nucleus seen in the movement and trace the patterns and differences of what writers are trying. We can say there's a line that reaches from Howard to Moorcock to Donaldson and Gemmell to Cook and Kay to Martin's Song and Erikson to Abercrombie, and look at it.
The "opposite" of grimdark doesn't really exist because these movements and sub-categories aren't really in opposition to each other. It's more fluid and thematic. But essentially, it would be comic/satiric fantasy -- Robert Asprin's M.Y.T.H. Inc., etc. that would be the most opposite a lot of the time. But bear in mind that comic/satiric fantasy as a general sub-category includes very dark satire and dark satire is frequently used in grimdark as a key element.
Is this helping or getting more confusing?