What is the relationship between the unit of Gibbs free energy and the spontaneity of a chemical reaction?

The unit of Gibbs free energy, which is joules (J), is used to measure the amount of energy available to do work in a chemical reaction. The spontaneity of a chemical reaction is determined by the sign of the Gibbs free energy change (G). If G is negative, the reaction is spontaneous and can occur without external intervention. If G is positive, the reaction is non-spontaneous and requires external energy input to proceed.