Consider the graph of Gibbs free energy G vs extent of reaction....

For spontaneous process, $∆ G = - ve$, at equilibrium $∆ G = 0$ and for non-spontaneous process $∆ G = + ve$.

The relation between the change in Gibbs reaction energy and Gibbs energy can be defined as the slope of the Gibbs energy plotted against the extent of reaction at constant pressure and temperature.

At point a: Slope$= - ve$

$∆ G = - ve$

Hence, reaction is spontaneous at point a.

At point b: Slope$= 0$

$∆ G = 0$

Hence. it is the equilibrium condition.

At point c: Slope$= + ve$

$∆ G = + ve$

Hence, reaction is non-spontaneous at point c.