1. In a sense there are no "lost" tribes since when Israel (the northern kingdom) was destroyed there were already members of all the tribes living in Judah. The main tribes in Judah were the Benjamites, the Levites and the Judeans but there were other tribes as well.

2. Israel was destroyed about 150 years prior to Judah but a Jew is a Jew is a Jew. The only way a Jew from Israel would be called a gentile is if that Jew had a gentile mother (and ergo was not a Jew).

So whoever told you that the word "gentile" means "Jews from Israel" is full of beans.

I have no idea if Paul would have called Hellenized Jews Greeks or not, but since Hyam Maccoby (amongst others) figure Paul was a Greek himself (and indeed he claims to be a Roman) how big an insult could he have meant it to be? After all Paul worked for the Sadduccees who were highly Hellenized.

Bottom line is it doesn't matter.
And everything that Sarah tells you, listen to her voice. Genesis, 21:12