The formula for the change in Gibbs energy of a solid is:
ΔG = Vm ΔP
where, ΔG is change in Gibbs, Vm is molar volume, ΔP is change in pressure
ΔP = P(final) – P(initial)
P(final) = 1 atm = 101325 Pa
P(initial) = ρ_water *g *h = (1030 kg/m^3) * 9.8 m/s^2 * 2000 m = 20188000 kg m/s^2 = 20188000 Pa
Vm = (950 kg/m^3) * (1000 mol / 891.48 kg) = 1065.64 mol/m^3
So,
ΔG = (1065.64 mol/m^3) * (101325 Pa - 20188000 Pa)
ΔG = -21405164347 J = -21.4 GJ