Physical significance of magnitude of ΔG (change in Gibbs free energy) of a reaction
It is only a rule of thumb that a negative value for $\Delta G^0$ of a reaction means that a reaction is spontaneous.  If you start out with pure reactants (and no products present), all reactions are spontaneous, irrespective of the $\Delta G^0$.  A negative value for $\Delta G^0$ only signifies that the equilibrium constant is greater than 1.