Why is Gibbs free energy 0 at phase change?

The Energia libera di Gibbs is not necessarily zero at a phase change.

However, since the Gibbs function depends naturally on the Thermodynamic variables #p# e #T# and since common phase transitions occurs at a constant #p# e #T#, thus, the molar/specific Gibbs function tends to be a constant during the phase change.

Thus, change in the Gibbs function is zero, i.e. #dG = 0# even though the molar/specific Gibbs function is non zero.