How did reducing the employment gender gap impact the US economy?
I'm eyeballing that the unemployment rate of man rose by 3% while the rate for woman fall by 13. Clearly not all of those newly working woman replaced man.  

You choose a rather long period with incredible changes to the US economy; The one I think responsible for the effect you saw being the shift from manufacturing to service.

The Situation in 1977 can be seen on Page 10 (4 in the original numbering) of the 1977 Census.
It shows 44% of all employees working in manufacturing and 39% in service (Retail + "Selected services industries".

Contrast that to the 1997 Economics Census (Page 20 or 14) where "Retail Trade" alone employed more people than manufacturing - and Services almost double compared to manufacturing.

That shift favored woman (and probably was only possible due to more woman choosing not be stay-at-home moms) which explains the diminishing employment gap.

(The methodology of those reports has changed considerably so the exact percentage rates can certainly not be compared -- but the shift is so big that the conclusion stands unchallenged)