And I'm saying guides kinda help answer that.
RTZ is semi-automatic, it will automatically RTZ someone they moment they attempt ill action against Gio or GER, even in situations they otherwise can't act. This is automatic, and happens without contact, killing, or what not. Of course, GER can use RTZ manually as well, but in regards to actions against them, it's automatic.
It's active as long as he has Requiem.
But a caveat, the death loop, he DOES need to kill, this is confirmed twice in JoJoveller and it even confirms that GER itself killed Diavolo as the initial death not the addict, reason being is with other things he simply says "no" which of course makes them not happen as you'd expect, but here, what he's doing is resetting one death, ad infinitum, thus, he needs there to be an initial death, as such his foe needs to die first or at least experience the act of dying, in which GER can have that reset infinitely so they die forever, but obviously he can't reset a death infinitely if no death exists for him to RTZ.