STACY - When the gospel and prophets say that the gathering will take place of the 10 lost tribes...which ones are they? I know Jacob had 12 sons: Reuben, Simeon, Levi, Judah, Issachar, Zebulun, Dan, Naphtali, Gad, Asher, Joseph, and Benjamin. Also, Jacob blessed Ephraim and Manasseh adopted them as his own.
Who are the lost 10 tribes? It states in the Mormon Doctrine the Ten Tribes comprising that kingdom captive into Assyria. I get so confused about all this. And every time I read my scriptures or try to study it always comes up in my mind to understand it better.The other two are the Jews and Gentiles still here known on the earth?

JOEL - Traditionally there were 12 tribes of Israel, however, some do not number Levi among the tribes (see Numbers 1:47-49) because his posterity was given the Levitical Priesthood to minister to all the tribes (see Numbers 3:12-13) and they held no tribal land. Joseph was given a double portion through his sons Ephraim and Manasseh. Thus, the twelve tribes of Israel include, as you listed above, ten of Jacob's sons and two of Joseph's (13 if you count Levi).

Things get a little complicated as to which tribes were "lost". The tribe of Manasseh was subdivided into two groups or half-tribes, one of which was called Machir (See Num. 32: 40; Josh. 17:1).
The twelve tribes were united under King David and his son Solomon, but ten of the tribes rebelled against Solomon's son Rehoboam and set up their own kingdom, which became known as the Northern Kingdom of Israel. Judah and Benjamin remained in Jerusalem and became known as the Southern Kingdom, or Kingdom of Judah. Simeon and Levi became assimilated into the kingdom of Judah. Actually, there were remnants of all the tribes found among the people of Judah. (See 1 Kings 12).
The tribes of the Northern Kingdom were taken into captivity by the Assyrian king Shalmaneser about 721 B.C. (See 2 Kings 17.)
They are the following, which we call the lost tribes:
1. Reuben
2. Asher
3. Ephraim
4. Dan
5. Issachar
6. Half-tribe of Manasseh
7. Half-tribe of Manasseh (Machir)
8. Naphtali
9. Zebulun
10. Gad

The rest of the tribes not included in the Northern Kingdom were Judah, Benjamin, Simeon, and Levi.
Generally speaking as far as Latter-day Saints are concerned, the lost tribes are Israelites other than either the Jewish people or the Lamanites of the Book of Mormon (2 Ne. 29:13).
Many have speculated as to where the lost tribes are but there is no official opinion about this from the Church.
You can read a good article about the lost tribes at the Church website:
John A. Tvedtnes, “The ‘Other Tribes’: Which Are They?,” Ensign, Jan 1982, 31