That novel, Outlander published in June of 1991, won a RITA award for best romance novel of 1991. However, the novel is more than romance. It is historical, science fiction, and fantasy. Its depiction of Scottish clan life in the 18th century is detailed and by all accounts extremely realistic. The novel marked the beginning of a series in which there are now 7 books, each a whopping 650 to 800 + pages. They are not trite, nor quick brain candy reads that many equate a romance with. These are great books that get you thinking, keep you interested and focused.
I've met all sorts of people who have read Gabaldon's books, doctors, business people, store clerks, students, teachers, you name them, they've read them. I think that what sets Gabaldon apart from typical romance writers is the incredible quality of writing. Yes, I read romance once in awhile. I like to escape with a fun novel, it's like watching a movie. But Gabaldon's novels aren't your typical escape novels, nor are they your typical quick reads. There are lots of details to remember, the quality of writing asks you to stop and savour the setting and to notice the features of the characters.
These are well-written books that are hefty and meaty. They last more than a rainy weekend, and I'm glad they do. They just take so darn long to come out in print, so don't get too hooked. 7 novels in almost 20 years. If you read faster than that, it's a long wait.