Unisex (undifferentiated as to sex) would often be suitable, but perhaps not in OP's context. Partly because it would imply a single [application of such theories] applicable to both male and female, and partly because it wouldn't sit well next to cross-national.
I suggest rewording to something like...
This research has been focused on applications of such theories that transcend the boundaries of nationality and gender.
EDIT: Apparently OP's context is concerned with differences across those boundaries, rather than "neutral/unaffected by" those boundaries. In which case focus on/exploit/whatever could be used instead of transcend. Wiothout knowing exactly why the differences are significant, I can't say which word would be best.