Don't know this giorno thing, put his feats in the first post lets see what he can do.
His stand (power basically) allows ANY damage on him to be completely negated and the time will reverse to before the damage happened. And it's not something that can be prevented since this is a stand that exists outside the normal plane of space/time. In the manga, it's shown to be existing in a state of "skipped time" so even stopped time cannot harm it. And if the GER stand manages to kill you then you will be put in an infinite death loop forever.
Return To Zero
Gold Experience Requiem's ultimate ability is to revert all actions and willpower back to the state of "zero", completely nullifying them and preventing them from becoming "real". For simplicity's sake, it's comparable to performing Ctrl-Z (undo), completely denying any causality. This can be seen as a direct opposite to King Crimson. As instead of undoing the act and leaving the effect like King Crimson does, Gold Experience Requiem removes the effect from the cause.
Death Loop
If the opponent is killed by Gold Experience Requiem, then they will continuously experience death, as they will die but repeatedly return to point "zero"; the point immediately before the process of death (i.e Diavolo's infinite death loops). Through this ability, GER creates a new "reality", as it calls it, for Diavolo as a form of poetic justice for the multiple times the mob boss has manipulated reality to his advantage with King Crimson.
It also regains all the abilities prior to the evolution so it can "grant life" to other materials.
@Fujishiro OT: I'm sure no one in the verse can even damage Giorno since no one has hax negation (on the level of how advanced GER is at least). Teach's DF negation is the closest that can come to this but I don't see it working on a stand that exists in a higher dimension. With that being said, the only AP feats we have on it is on Diavolo who's dura/endurance, while still amazing for normal humans, is nowhere even close to that of even vet level characters in OP. His stats are said to be infinite but I don't really know how much trust you can put in stand stats which are infinite. So Stand vs OP character, I see it to be a draw or GER winning due to exhaustion of the opponent. If you factor in Giorno's IQ which is magnificent then he can definitely win a lot easier vs these characters. Definitely not losing though since no one can bypass RTZ.