Confusion between various formulas of Gibbs free energy
The Gibbs free energy of a multicomponent mixture is a function not only of temperature and pressure, but also of the number of moles of the various species in the mixture:  $$G=G(T, P, n_1, n_2. m_3. ...)$$So, $$dG=\frac{\partial G}{\partial T}dT+\frac{\partial G}{\partial P}dP+\frac{\partial G}{\partial n_1}dn_1+\frac{\partial G}{\partial n_2}dn_2+\frac{\partial G}{\partial n_3}dn_3+...$$or
$$dG=-SdT+VdP+\mu_1dn_1+\mu_2dn_2+\mu_3dn_3+...$$ where the $\mu's$ are the partial derivatives of the Gibbs free energy with respect to the number of moles of each of the species, and are referred to as the so-called Chemical Potentials of the various species in the solution.  The numbers of moles of the species in the mixture change as the reaction proceeds, and that causes the Gibbs free energy to change.