For a spontaneous reaction the $\mathrm{\Delta G}$, equilibrium constant $\left(\mathrm{K}\right),\text{ and }{\mathrm{E}}_{\text{cell }}^0$ will be respectively

The standard free energy related to the equilibrium constant Kc as 

$$\begin{gathered} △{\mathrm{G}}^{\mathrm{o}}=-2.303\hspace{0.17em}\mathrm{RT}\hspace{0.17em}\log \mathrm{Kc} \\ ∆{\mathrm{G}}^{\mathrm{o}}=-{\mathrm{nFE}}_{\mathrm{cell}}^{\mathrm{o}} \end{gathered}$$

If a cell reaction is spontaneous ( proceeding in forward side ) , it means 

$$\begin{gathered} \mathrm{Kc}> \hspace{0.17em}1 \mathrm{and} {\mathrm{E}}_{\mathrm{cell}}^{\mathrm{o}}= +\mathrm{ve} \\ △{\mathrm{G}}^{\mathrm{o}}= -\mathrm{ve} \end{gathered}$$