why is gibbs free energy minimised when a system reaches equilibrium???

Question

assuming it is an isobaric process

Answer 1

Becasue that's kinda the -definition- of local equilibrium for any phase-space system. And note that I very carefully said 'local' equilibrium. It's quite possible (in many systems) that the addition of some small amount of energy (usually called 'activation energy') can cause the system to travel to an even lower energy state by giving off more energy in the process.

HTH

Doug