A quick look at the tables of thermodynamic values will show their use is very restrictive. The temperature must be 298 K, all concentrations must be 1 M and all gas pressures 1 atm. Since these condition are seldom met, we are generally using ΔG and it changes during the course of a reaction just as Q does!!!!

Here is how it all fits together:

ΔG = ΔG^o + RT ln Q_c As a reaction approaches equilibrium, ΔG approaches 0 and Q_c approaches K_c until:
0 = ΔG^o + RT ln K_c OR ΔG^o = -RT ln K_c Look familiar?