Okay, let me put it this way:
GenServer is a building block. You can use it to implement your own scheduler, yes. It does not come with out of the box functionality to run task ever X amount of time, you have to program it.
Quantum is a ready to use scheduler that you just configure.
Go with GenServer and you will need to write more code but the end result may be better tailored to your needs. Or be more unreliable and buggy compared to battle proven, existing scheduler libraries.
Go with existing scheduler library if you don’t want to write all the boilerplate/scheduling code yourself, want battle proven solution from the start and willing to work around any limitations you may encounter.
Having said all the above, I would go with quantum first.
