I think it was during the Reagan era.
My recollection is that in 1976, women favored Ford over Carter, so that year, the gender gap went in the other direction. Prior to that, I don't think there was a discernible gender gap, at least not that anybody talked about.
Prior to the 1980s, I don't think politicians really considered women a separate voting bloc. It was assumed that they would vote their economic class, just like their husbands.
I think the media in general gets the gender gap all wrong. The gender gap came about as a result of men leaving the Democratic Party as it became perceived more and more as the part of wimps. This was especially true in the contrast between Carter and Reagan, and has remained true since.
Democrats, buying into politically correct thinking about "women," began targeting women specifically with some success. But overall, the gender gap has been a bigger problem for Democrats who are unable to attract male voters than Republicans. Remember, there is a lot more shame in a man acting like a woman than there is in a woman acting like a man. And in some circles, being a Democrat has a somewhat feminine cast to to, like driving a VW Cabrio.
There was some mention of a gender gap back to the 60's. The initial mentions I recall were on the impact of television. Some pundits speculated that telegenic candidates (of course all male at the time) like JFK might be more favorably viewed by women. I was young, and I don't recall if any data was ever produced to support those speculations.
