$2$ moles of xenon and $1$ mole of neon are in two separate sides of a container with a wall separating them at $25C.$ The wall doesn't move and forces the gases into two sides of equal volume. Because of this, xenon's pressure initially is twice the pressure of neon. When the wall is removed, the gases expand, and the total pressure is the average of the two initial pressures. Calculate the Gibbs energy of mixing for the two gases.