After watching all of Part 5, here's my thoughts on Giorno's ability:
His power is whatever is currently required by the plot. He can send a person's consciousness out of control when he has no other way to beat them. He can have his stand extract an antidote for a virus from a snake's blood, and transform objects into functional body parts which heal seamlessly and immediately. He can detect life force to determine the amount of souls in a body.
And the Requiem literally rewrites the big bad's fate with a death loop.
tl;dr: Giorno has the offensive equivalent of plot armor