The Carnarsie Neighborhood of Brooklyn, (according to Wikipedia) was predominately Italian/Jewish from the 1920s until the 1980s/90s. Seeing as a lot of people used to meet, marry and stay in the neighborhoods they were born in, it's not a stretch to think that Bobby's parents were both from Carnarsie.
From Answers dot com:
Jewish (Ashkenazic): altered form of
Horn 5, under Russian influence; since Russian has no
h and alters
h in borrowed words to
g. In Israel the name has been reinterpreted by folk etymology as being from Hebrew
goren 'threshing floor', which is in fact etymologically and semantically unrelated.
Judging from the differences of appearances between Bobby and Frank, I'm guessing that Poppa Goren was of Slavic decent, since Bobby obviously takes after his mother when it comes to his coloring. Frank (and Tony Goldwyn) is paler, with blonder hair and blue eyes.
Whether or not Poppa Goren was Jewish is a tossup, but it wouldn't make Bobby Jewish because that's passed through the mother, not the father.
Actually, knowing how tall Poppa Goren was might settle the debate on the whole Brady/Goren/TodayOnMaury! issue that Bobby has. Frank obviously inherited his mother's height, and Brady wasn't a big man either. If Poppa Goren were a larger man, Bobby might have inherited his height from him. (Of course I'm not figuring grandparents genetic donations, because I inherited my grandmother's height, which makes me the shortest one in the family.)