A wonderful explanation by All difference between follows:
1. $\Delta$ G relate to these two quantities ΔH° and ΔS° while $\Delta$ G° does not relate to the quantities:
$\Delta$ G definitely does relate to those two quantities. ΔH° and ΔS° represent the change in enthalpy and entropy between product and reactant but they do not mean a “100% complete reaction.” They determine the energetic difference (at a given temperature). This difference in energy determines the composition at equilibrium
2. $\Delta$ G is not a non-zero value while $\Delta$ G° is a non-zero value:
$\Delta$ G° is a non-zero value and it can’t use ΔH° or ΔS° to find $\Delta$ G because either ΔH° or ΔS° represents 100% complete reaction.
3. At equilibrium $\Delta$ G is equal to zero while $\Delta$ G° is negative:
If $\Delta$ G° is negative at equilibrium, then we will have lots of products at equilibrium, which means that Q must be greater and also greater than 1 to approximate K. When Q gets bigger, it means more product is accumulated. The term ‘$RT \ln Q$’ gets increasingly positive, and eventually adding that term to a negative $\Delta$ G°, will make $\Delta$ G = 0, equilibrium will be established and no further change occurs.
4. Equilibrium is established when $\Delta$ G is at zero while lots of reactant at equilibrium when $\Delta$ G° is positive:
If $\Delta$ G° is positive at equilibrium, then we will have lots of reactants at equilibrium, meaning Q needs to be smaller (less than 1) to approach K. As Q gets smaller (i.e., as we get more reactants), the term ‘$RT \ln Q$’ gets increasingly negative and eventually adding that term to a positive $\Delta$ G°, will make $\Delta$ G = 0, equilibrium will be established and no further change occurs.
5. When $\Delta$ G is zero at equilibrium, it will define which way the reaction proceeds while $\Delta$ G° does not:
Note that it is $\Delta$ G and not $\Delta$ G° that will be zero at equilibrium and the sign of it generated by the combination of $\Delta$ G° and $RT \ln Q$, will define which way the reaction proceeds.