I believe the main issur is formally teaching Gemora to them. (As in a class, as opposed to an exceptional yochid. The ???”?, Tuv Ayin, 4 holds such a yochid can be taught [not sure if its referring to baal peh in general or even Gemorah]. But others disagree with the ???”?, saying even a yochid shouldn’t learn it.)
Is Rav Nosson considered part of the Chareidi world or the YU world? YU has their own liberalized shittos, as classes teaching women Gemorah. (Which has no source prior to Rav Yosef B. Soloveitchek.)