Is constant pressure heat capacity = entropy during adiabatic reversible expansion of perfect gas?
Inspired by Chet Miller.
The fallacy in the argument is that, ΔG = 0  because the process is reversible. This is not true and so leading to wrong final result.
ΔG is a signpost for spontaneity of a process only if the process occurs through constant temperature and pressure. Likewise, that ΔG = 0 means reversibility, is true only if the process happens through constant temperature and constant pressure.
Adiabatic expansion clearly involves change in temperature, so ΔG is not a signpost for spontaneity of adiabatic change, and hence ΔG ≠ 0 for reversible adiabatic expansion.