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 Quote by AntiElephant http://img842.imageshack.us/img842/7200/img0103iu.jpg The relation $dG = -SdT + VdP$ Is derived by considering the defition of Gibbs free energy. However, this formula is based on state variables, and hence works for all transformations. Right?

Don't know if you already solved your problem but it seems simple to me. There is an error in your statement "and hence works for all transformations", it does not work for all transformations it only works for reversible transformations. In case the transformation is not reversIble the correct equation is $dG ≤ 0$, where P is the external preasure and T is the source temperature that are supposed to be constant, the preasure and temperature of the system not even defined por a non equiblibrium state