Obviously, I disagree that books by women make poor gateway books and see no reason why, say, Phyllis Gotlieb's Sunburst (1964) should not do the trick just as well as any other sf novel of the 1960s (assuming you were determined to pick one from that decade).
On a more general note (ie, not specifically in reply to JD): bear in mind that Margaret St Clair had 80 stories published during the 1950s alone, all in major titles such as F&SF, Galaxy and Startling Stories. And yet now she's almost forgotten, unlike many of her male contemporaries. Women have been writing sf since the genre's beginnings, they've just been written out of the genre's history, and all-male lists perpetuate that. So-called "gender-blindness" fails to address that and, in fact, only makes it worse - because you're relying on subconscious biases instead of making them conscious and working to counteract them.
You make some good points, Ian; and, while I would still argue that, from my own reading, I have encountered fewer "gateway" books by women than men, I like your suggestions here and below. Certainly, I think it is darn near a crime that Margaret St. Clair has been forgotten, and I would also agree that there is a strong tendency still (or perhaps that should be again, as it seemed to be fading for a few decades) to see sf as largely a "men's preserve", when we have had a considerable number of fine women writers as well.
As for the suggestions you made, I would heartily second Sargent's three
Women of Wonder anthologies (which, unless memory deceives me, have been collected together into a single tpb several years ago). I also should have mentioned (and, to use your phrase, this is an obvious choice) is James Tiptree, Jr., surely one of the truly original voices out there, in my earlier list....
I will admit that I didn't list, say, Andre Norton because I've read very little of her sf, though I've read a fair amount of her fantasy; hence didn't feel qualified to say anything about her work. However, from my understanding, much of her work in the field would certainly apply at least as well as, say, Heinlein's. As for the Brackett, Cherryh, and (especially from my point of view, considering how highly I consider her work) Moore, I think yes, I deserve something of a rebuke for not mentioning those in this context, as they would certainly qualify. (For that matter, just about anything Moore had a hand in is worth looking into; and Brackett, though not among my favorites, produced a lot of fine sf during the "classic" period....)