To calculate $\Delta G$, first obtain $\Delta H$ and $\Delta S$ and then use: $\Delta G = \Delta H - T\Delta S$, valid for an isothermal process.

Another alternative is to integrate the equation, $(\partial G/\partial P)_T=V$. \begin{equation} \Delta G=\int_{P_1}^{P_2}VdP \end{equation} In the particular case that P and T are constants $\Delta H = q$ and $\Delta S = q/ T$, therefore $\Delta G =0$. An example of this situation is the reversible phase change.

The calculation of $\Delta A$ is done with the expression $\Delta A = \Delta U - T\Delta S$, or with $\Delta A=-\int_{1}^{2}PdV$