Why do we use gibbs energy instead of considering the total entropy change when we are determining whether a process will occur?

Because with Gibbs free energy we can determine the spuriousity of reaction occurrence

Explanation:

Gibbs energy: is a thermodynamic potential that has units of energy that gives the condition of equilibrium and spontaneity for a chemical reaction (at constant pressure and temperature).

In the chemical thermodynamics of reactions and in non-cyclic processes, there are four useful quantities called "thermodynamic potentials". These are internal energy, enthalpy, Helmholtz free energy and Gibbs free energy.

While Entropy is a thermodynamic function that is a measure of the unusable part of the energy contained in a system or matter. It serves to measure the degree of disorder within a process and allows us to distinguish useful energy, which is what becomes entirely work, of useless, which is lost in the environment.