Dear Student, Gibbs free energy is a measure of spontaneity in a reaction. (a) In a galvanic cell that is spontaneous, Gibbs free energy decreases as the cell moves towards a more stable equilibrium. It is the useful work done by the cell. It is with negative sign. b) In an electrolytic cell, energy is added to the reaction (like charging a battery.) In this case there is increase in the Gibbs energy. It is the useful work done on the cell by external power. It is with positive sign. Regards