In the case of $f_c(z) = z^2 + c$, [this][1] recent preprint shows that we can factor $g_n(c) \pmod 2$ into irreducibles of degree $n$ in the odd case and something similar works for $n$ even too. In particular, see Theorem 1.2 and Theorem 1.6.

Unfortunately, the techniques don't seem to generalize in a straightforward manner for arbitrary $f_c(z)$. They do seem to handle the case of $f_c(z)= z^{p^n} + c$ for $p$ a prime however.


  [1]: http://www-personal.umich.edu/~kochsc/GleasonMod2.pdf