In particle physics, G-parity is a multiplicative quantum number that results from the generalization of C-parity to multiplets of particles.
On the other hand, strong interaction does not see electrical charge, so it cannot distinguish amongst π+, π0 and π−.
We can generalize the C-parity so it applies to all charge states of a given multiplet: where ηG = ±1 are the eigenvalues of G-parity.
G-parity is a combination of charge conjugation and a π rad (180°) rotation around the 2nd axis of isospin space.
In general where ηC is a C-parity eigenvalue, and I is the isospin.