The surname Gullo did not turn up on my favorite site
Gens - Italian names etc
[Sigh] Just checked surnames at Gens and the map shows many Gullo families, especially around Palermo (should ask Maria about that area):
Gens - Gullo Surname map
Here's a Palermo marriage Index:
Palermo Marriage Index - rootsweb
I did have a Palermo Birth Index but it is no longer found.
Another had many for the map section:
Italian Family Names
Check out the Fake Italian Names and of course, baby names.
Usually I find good info at Italy World Club but their G list only went as far as Gr...
Italy World Club Genealogy
They do have many communes and frazioni with lists of surnames in those places.
Have to check some other places later. (Did check and edited)
Lots by Google search including this one:
Gullo Surname History