Gamin(e) literally means a mischievous kid, like a street urchin. So in fashion/ style terms, to me it's always connoted a cool ('effortless') tomboyish (adolescent-boy) quality, usually applied to small-framed lithe women. Short hair helps.
Jean Seberg (in A bout de souffle or Bonjour Tristesse) is probably the paradigmatic example of the gamine for me. The gamine (like Seberg) can be very seductive, even a coquette--- but not in a frilly girly attention-seeking way... more insouciant & sly... the image that comes to me is, like a playful cat.
Audrey Tatou's a good contemporary example. I'd throw in, say, Clemence Poesy or Lou Doillon-- who, even with long hair or wearing very sexy dresses, still exude something of the cool beautiful teenage boy. (Charlotte Gainsbourg, in earlier incarnations, seemed gamine to me-- but now she seems too tall & too strong & adult, in a way, to be 'gamine'-- which has an element of 'cute' & 'small' & 'childlike'.)
The original character of Lolita as described in Nabokov's novel has something of the gamine (Nabokov calls it "nymphet"), but of course now the term "Lolita" for us (in pop culture & fashion) connotes something distinctly different from the gamine (more self-consciously sexual &/or girly, say).
Audrey Hepburn is very much the gamine... eg in Sabrina or Funny Face or or Breakfast at Tiffany's etc.... but the important thing is, the gamine quality is precisely *not* the "sophisticated" side of those characters-- but the fact that, even at her most chic & sophisticated (wearing couture creations), she still retains something of the tomboyish girl she was/is-- who climbed & hid in trees (Sabrina), or was a wild homeless untamable child (Holly Golightly), or defiant tomboy (Funny Face). That's why Jackie is *not* gamine-- she's sophisticated like Audrey, but one doesn't see the playful (tom)boy in her.