There are several thermodynamic 'potentials', all with units of energy.
The Gibbs potential (G) is commonly used because it is the one which is minimised when a system at constant temperature and pressure reaches equilibrium.
I guess you have seen this, but G is defined as:
G = H - TS
Where H is the enthalpy, T is the temperature and S the entropy.
You can see that minimising enthalpy and maximising entropy (at a constant temperature) will both mimimise the Gibbs free energy.
For spontaneous processes (at constant temperature and pressure), the Gibbs free energy does decrease, which can be due to a reduction in energy, an increase in entropy, or both.