The G5RV in its standard form is not a balanced antenna. There is a transition from open wire feeder to coax. Which affects the balance considerably. Due to common mode current flow on the outside of the coax screen.
Sounds like a G5RV installation that ignored use of balun at feedpoint is all.
Same goes for *any* dipole installation, coax or balanced line fed, it is indeed a balanced antenna in the sense that it does NOT need ground radial counterpoise in order to work as intended.
Half of it is indeed the "counterpoise".
I've operated way too many dipole antennas of all types with no ground whatsoever. G5 inclusive.