DG = DG^0 + RT ln K (I used D instead of delta). Both DG and DG^0 describe the equilibrium reaction and we say that equilibrium the gibbs energy value is zero, then why are not both zero and we take only DG as zero?

How can an equilibrium reaction A<====>B having delta G = -x allow for the backward reaction to occur at a time of the reaction if then delta g would be +ve?