3 Nephi 16

 The Savior explained what he means of gathering in the last days as He tells the people in the Americas - 

"I command you that ye shall write these sayings after I am gone, that if it so be that my people at Jerusalem, they who have seen me and been with me in my ministry, do not ask the Father in my name, that they may receive a knowledge of you by the Holy Ghost, and also of the other tribes whom they know not of, that these sayings which ye shall write shall be kept and shall be manifested unto the Gentiles, that through the fulness of the Gentiles, the remnant of their seed, who shall be scattered forth upon the face of the earth because of their unbelief, may be brought in, or may be brought to a knowledge of me, their Redeemer."  (3 Nephi 16:4)

"Brought in" or gathered is those that possess a knowledge or testimony of their redeemer.