Relationship of free energy (G) with the equilibrium constant (Keq)?

I'm not exactly sure what you"re asking, but ΔG=-RTlnK. At equilibrium, when the constant K is equal to [products]/[reactants], ΔG+RTln(Keq)=0. The equation that you wrote, is it supposed to read A+B=C+D, or something similar to that?