gianni mangia una mela, gianni, gisella ha fatto i toast, gisella e mariangela buonarro(s)ti are all written with a lowercase letter at the beginning.
It is recommended to write these names and phrases between quotes ("gianni") or in italics (gianni), to violate spelling rules more gently.
You may write just mariangela buonarro(s)ti's name (mariangela), but not just her surname (buonarro(s)ti).
buonarro(s)ti is always written with the s between parenthesis, not buonarroti or buonarrosti, but it is pronounced buonarrosti.
The fruits (apple, pear, peach, ...) are gianni's. Examples: gianni's apple, gianni's pear, gianni's peach.