A listing of some theories for the name Γκατζογιάννης that I’ve heard and researched myself. Specifically, the decidedly non-Greek γκατζο part.
Yiayia’s Story#
One time I asked my Yiayia about what the name meant. According to her, it was the name of a famous chicken in a nearby village. To this day I never found out if she was joking with me or not.
Nick Gage’s Serbian Theory#
According to Nick Gage, Γκατζο comes from an old Serbian dialect, meaning “black,” with the name meaning “black John.”
Romani#
In Romani, Pan-Vlax, etc., gadjo/gadzo/gaʒo is a word for a non-Romani man (and depending on the regional variation, can also mean a Serg, Christian, peasant, etc.). See gaʒo, gajo, gadzo.