Why Gibbs energy change of a process can be negative at constant temperature and pressure? This question has been asked before, but the answers are not clear and satisfactory.
If the pressure and the temperature are constant then G would be always zero. How would then G have a value other than 0?
G is a function of T and P, but also, in a mixture, the number of moles of each of the species in the mixture.  If these change because of a chemical reaction, G also changes (even at constant T and P).