I don't see how he wasn't. It's like saying voice acting changed Joseph Joestar. They didn't add sudden bouts of emotions for Giorno, they were always there.
He also killed Polpo in cold blood in the manga
Oh yeah of course, I'm not talking about BIG changes, but more subtle inflections and reactions. It's also hard to convey "charisma" through text, and the voice actor pulls that off perfectly. There's also a couple of small art tweaks for some panels that better show things like Giorno smiling more in the airport and showing more concern for the Mario janitor. Not saying these are faults of the character at all, but moreso the limitation of the manga medium and the benefits of having a whole production put into this adaptation.
Literally the only added scene for Giorno is him stealing from the girls. Everything else is from the manga.
Edit: oh and the scene of him watching his rolemodel get confronted by the drug dealer's kid
Well, these "two" scenes added:
-Giorno greeting the two women, getting the wallet back, and getting some of their money himself. This reinforces Giorno's friendly AND deviant side, and shows he's not above protecting others for a service fee.
-Giorno walking through alleyways of drug addicts, drug addicts, and criminals. This immediately establishes the setting, the crime and gangster riddled world of Vento Aureo. Seeing Giorno disgusted and concerned already tells us that Giorno has lived this through hellhole and gives us the "why" to his motivations. Seeing how Italy is corrupt from his perspective makes us empathize with the character and understand how much he wants to change things.
-Giorno giving the kid ice cream. As small as this is, this actually adds a MUUUCH softer side to Giorno that we've never seen at all from Giorno in the manga. This also reinforces Giorno's care for children and why he doesn't want drugs to reach them.
-The role model Gangstar buying ice cream for him. This ties directly back to the previous point and links his backstory with his current motivations. Now we know the direct influence of WHY Giorno realizes it's so important to protect and guide kids, considering he was straightened out himself.
-Giorno witnessing the role model Gangstar killing in cold blood. Why is our Shonen protagonist a cold blooded killer? Well, the person he modeled himself after showed him that he's not above killing for a greater cause. This scene shows us and Giorno that being a gangstar not only entails JUST being in the mafia and risking your life, but it also involves making difficult choices that'll end up hurting others.
-The role model defusing the the situation with the drug dealer's kid and saying his father was a bad man that deserved it. Same deal as above, and someone here also pointed out that this is a pretty damn good parallel with what's going on with Trish. Now Giorno himself has to do the same thing his role model did years ago.
Come on guys, I'm not even saying that manga Giorno is bad at all or lacking in any of these points, I'm saying that the anime version is notably BETTER, which I think we can all agree on.