If you have two parallel planes with one density of 1 C/m^2 and the other with -1 C/m^2 density and you need to find the electric field at a point in the middle, how do you use Gauss's Law?

I drew a box enclosing one plane to find the electric field in the midpoint. I understand there's another plane exerting an additional force, but by Gauss's law it should only depend on the enclosed charge.

Is Gauss's Law being violated here?