there's a physical change between the gios that everyone sees. They aren't completely assimilated and so the way the other gios features pop up is through physically over taking the body.
The collectors guild mentioned earlier that Jio wasn't hiding the fact that he wasn't human because he was changing his appearance in front of them so they can see the physical changes too, hence why the little boy commented on Gio's "beautiful blue eyes" when Jio has dark grey/black eyes
It's visible to everyone. Anyone he's changed in front of has been able to see Giovanni.
In the manhwa, we the reader can see that when jio changed his personality to giovanni, we can see giovanni's in person. But is he still look the same for the ppl around gio?? I'm still wondering