You’ve discovered the difference between G4Isotope, which is part of the G4Material framework for defining geometry elements, and G4Ions, which are active trackable particles corresponding to nuclei.
You can make your geometry out of anything you want. It won’t decay. It won’t get activated. It’s absolutely passive stuff which is identical from event to event in your simulation. It’s the “ground” within which all the tracks do their thing.
If you want to generate events simulating Co-60 decays, then what you want to do is create a Co-60 G4Ions as your primary particle (i.e., in your PrimaryGeneratorAction), and G4 will have it decay, and track the resulting secondaries, etc.
If you are using G4ParticleGun, you can do this with two macro commands:
/gun/particle ion # This says you want to create a G4ions primary
/gun/ion 27 60 # This says you want Co-60 as your primary
If you’re using GPS, you can do the same, but with /gps/... commands instead.