In small towns, to distinguish one guy from another with the same first name, 'di' was added relating to a one word description of the father or of the family, or of the original city they came from (in this case 'da' = from). If the parents have in their family two names that are relevant for them, they can be merged for the new-born like in Gianfranco (Giovanni+Franco), Pierpaolo (Piero+Paolo); you will also find the form Abc-maria (examples: Gianmaria, Annamaria), which is related to Christian religion; 'di' is called 'patronimico', that is referring back to the paternal origin. Other family names like degli Angeli, De Angelis tell a story of somebody in the ancestor line that has been probably abandoned in front of a church and therefore initially raised with the help of 'angels'; the same probably for Donati (given as a gift). Many family names appear as plural, I would call that a false plural, and actually the desinence of genitive in Latin, therefore still 'patronimico'; a clear example: Rutili (from rutilus== red, in Latin). Rutili: of the red (person).

Hi, the name Gian stands for "Gianni" and the name Gianluigi is a counpond name which consists of 2 names: Gianni and Luigi. The desinences "Di", "Del" doesn't have a particular meaning in the surnames. They are translated as "of", for example "of rossi".

Gianluigi is a compound name. Gianni (it) = John (en) = Jean (fr) ... Luigi (it) = Lewis (en) = Louis (en, fr) ... Gianluigi = John-Lewis = Jean-Louis It seems to me that compound names (e.g. Giambattista Basile, Jean-Jacques Rousseau) are relatively more popular among Italian/French-speaking nations than English-speaking nations. "Di" and "Del", as valeria and Andrew has well depicted, indicate the origin of the name bearer's family.