For the methane combustion reaction (CH4 + 2O2 → CO2 + 2H2O),
ΔH = -1039 kJ/mol and ΔG = -1039 kJ/mol.
How much energy is generated as heat from this reaction and how much energy is wasted to increase the disorder of environment? Then, how much energy is available for a useful work.
At this end of these questions you will understand why ΔG is the maximum useful work to be available from spontaneous reaction. Then, how would you say ΔG if reaction is nonspontaneous?