It's interesting that modern Gk can have the plural here. I had never thought of it before, but the ancient philosophers also could use the plural for general knowledge, as Aristotle does in Met. 981B καίτοι κυριώταταί γ' εἰσὶν αὗται τῶν καθ' ἕκαστα γνώσεις. The Septuagint translation sometimes uses plural because the Hebrew has plural, as at 1 Sam 2:3, θεὸς γνώσεων κύριος, but the plural would not have been jarring to Greek ears. Plato sometimes used the plural γνώσεις for general knowledge (αἱ γνώσεις πᾶσαι) and also epistemai. At Paul's 1 Cor 13:8, for the singular γνῶσις majority reading some manuscript read γνώσεις.
"Their knowledge" would be οι γνώσεις τους, regardless of the gender of "they" (all male, all female, mixed, or even neuter).
See e.g. http://ebooks.edu.gr/modules/ebook/show.php/DSGYM-A112/621/4006,17974/ , section 5.1, weak form genitive of third person plural pronouns.