Edit for clarity: If you measured delta G for a reaction at 1 M concentrations, 298 K, and 1 bar pressure, you would get a certain value. That's literally the same value you would get from evaluating $\Delta H_\mathrm{std}-T \Delta S_\mathrm{std}$ standard after substituting in $\Delta H_\mathrm{std}$, T, and $\Delta S_\mathrm{std}$. I was under the impression that $\Delta H$ and $\Delta S$ didn't change super significantly with concentration, so you could probably also use $\Delta H$ and $\Delta S$ values at any random concentrations to find 1 M Gibbs free energy. I don't understand what's special about 1 M Gibbs free energy that makes that work.
If you're given $\Delta H^\circ$ and $\Delta S^\circ$ values for a reaction, you can use $\Delta G^\circ=\Delta H^\circ-T\Delta S^\circ$ to calculate standard Gibbs free energy change for that reaction, which by definition is Gibbs free energy change when reactants and products are 1 M or 1 bar of pressure. And if you want to find Gibbs free energy change at nonstandard concentrations, you can use $\Delta G = \Delta G^\circ + RT \ln(Q)$. But here's the thing: I thought $\Delta H$ and $\Delta S$ (per mole) aren't really affected by the concentration of reactants and products. So if I gave you $\Delta H$ and $\Delta S$ values for when concentrations are 1 M, those values would be (essentially) the same as the $\Delta H$ and $\Delta S$ values at 2 M. In other words, my understanding is that $\Delta H^\circ $ (enthalpy change at 1 M concentrations) is essentially equal to the $\Delta H$ value at any random set of concentrations (same with entropy). So I could probably get away with saying $\Delta G^\circ=\Delta H-T\Delta S$ to get a pretty good approximation for $\Delta G^\circ$, even if the $\Delta H$ and $\Delta S$ values weren't technically "standard." But what's so special about 1 M that would allow me to substitute $\Delta H$ and $\Delta S$ values for any random concentrations into this equation on the right side, and still get Gibbs free energy change at 1 M concentrations on the left side, instead of, say 2 M concentrations? Unlike enthalpy or entropy, Gibbs free energy change is very dependent on the reaction quotient, so the Gibbs free energy change values at 1 M could be very different from those at 2 M concentrations (as long as, well, the reaction quotient didn't have the same number of moles of reactants as products).
