It’s possible that having the main qt (6) Formula installed is causing the build to pick up headers from this ahead of the qt@5 “keg only” that CMake found. Might be that /usr/local is appearing earlier in the include path.
I would try brew unlink qt and then try rebuilding Geant4 (you shouldn’t need to reconfigure, just rebuild). That should diagnose whether this is the case or not.
Note that we don’t test or support building/using Geant4 with GCC on macOS at the moment, so I’d go back to using the system Clang.