Thornyhold does have a certain amount of horror-peril, IIRC (though I don't 100% love the way it resolves), but you're right, it's also definitely primarily about the joys of being a plucky heroine in a charming cottage in an idyllic setting with just enough peril to keep things interesting for the length of a book. Not terribly gothic at all! But I go to Mary Stewart for the prose and the heroines and the poetic descriptions of scenery (and occasional intrusions of magic when she just can't help herself) more than the gothic plots, really, so that's fine with me.

I haven't read Rose Cottage or The Stormy Petrel yet, though, and now I am reminded that I should add them to the TBR list.