I had kind of just assumed that Madame Gao was an Inhuman and was meant to tie in slightly to the expansion of that mythos taking place through Agents of SHIELD. But some further research indicated something else.This article
spells it out a bit more - the packets of heroin that Gao was producing are stamped with a red snake, which is the symbol of an archnemesis of Iron Fist (getting a Netflix series as well). Specifically it's someone named "Steel Serpent"
, who is a native of a place called K'un-L'un, which is some kind of magical/technologically hidden city inhabited by dimension-hopping humanoid aliens.
The short version is that the working theory is that Gao is someone called Crane Mother