Heat Capacity temperature dependence and Gibbs energy
At constant pressure, $dG=-SdT$.  Also, the definition of G is $G=H-TS$, so $S=\frac{H-G}{T}$.  Substituting,
$$dG=\frac{G-H}{T}dT$$
This equation can be manipulated into the form:
$$\frac{d(G/T)}{dT}=-\frac{H}{T^2}$$
So,
$$\frac{d(\Delta G/T)}{dT}=-\frac{\Delta H}{T^2}$$
I leave the rest up to you.

A math error was corrected on 10/16/15