The Giant Under the Snow by John Gordon is another great children’s fantasy book from the late 60’s, like The Gruesome Green Witch and A Walk Out of the World, albeit only an Honorable Mention as far as the GU archetype goes.
Jonk (short for Jonquil, one of the more interesting GU names I’ve come across) wanders off from a school trip in the “backlands” one day and stumbles across a strange artifact and a Green Man figure embedded in the landscape. She is chased by a black dog and rescued by a mysterious woman named Elizabeth. Jonk and her two male companions learn more about the legend of the Green Man and are pursued by terrifying leather-skinned men when they seek out Elizabeth, who tells them the story of an ancient fight against an evil warlord who is trying to rise again. Jonk and her friends must hide the artifact from the adversary, but at least they get one big perk – magical devices from Elizabeth that allow them to fly! In the end, Jonk does end up facing off against the warlord one on one and thwarts his return.
Technically, I suppose this book fulfills many of the key plot points of a Girls Underground story, but ultimately something feels missing and it doesn’t quite fit the classic pattern. Perhaps it’s the lack of any meaningful interaction between Jonk and the adversary (he never even gets a chance to speak), the absence of a transition into another world, and her normal home life.