Notice "remnants" is plural. This is Christ speaking, and the scope of the message is universal. It is not local. It includes local events, to be sure. But the time of this fulfillment will be global. All the "remnants" will be affected.
It will not matter if the particular "remnant" is anywhere "upon the face of the earth" they will be "gathered in."
Why would they necessarily be "gathered?" What is the purpose of "gathering?"
Why "gather" merely to then return them to their lands of inheritance? (See discussion of 3 Nephi 21: 27-28 here.)
Which is more important, to gather physically or to gather "to the knowledge of the Lord their God?"
How could people gather "to the knowledge of the Lord their God?" What kind of "knowledge of the Lord God" will be involved? Do you get that knowledge by supporting men in their callings? Do men and their callings even matter? Can you grow in knowledge of God by following, even memorizing, a handbook; following, memorizing talks and inspirational literature? What does a person need to follow, to do, to abide by in order to gain "knowledge of the Lord their God?" What about those who testify to you about programs and personalities, but never preach about Christ and Him crucified?
Do true messengers speak about one another, or about their Lord? How can a man, any man, save you? Who alone has the capacity to redeem you? Is "knowledge of the Lord their God" related also to knowledge that He "hath redeemed them?" Can you "know" Christ and not acquire in the process of knowing Him the knowledge that He "hath redeemed" you?
Do you come to understand He has redeemed you by also coming to know Him?
Do Joseph's remarks about the Lord coming to visit with the remnant in the Rocky Mountains explain how both those coming from the four corners of the compass will gain "knowledge of the Lord their God" and also know He "hath redeemed them?"
Do you begin to see a pattern of consistent prophetic foreknowledge of the last days? Do Christ's words in this message of the Book of Mormon give any greater reason to believe in the promises?
If these promises are made by Him, should you expect it possible for you to go ahead and "gather in" to Him even before there are others willing to do so? Can this "gathering in" occur in your lifetime, for you? If God is no respecter of persons, then what would you need to do today to obtain the same blessings others will receive as they "gather in" in perhaps greater numbers in the future? Is it possible to do that? Are you willing to try?
It seems to me this doctrine is important in a macro sense in understanding prophetic promises and future gatherings. But it is perhaps more important in the micro sense, in that anything promised to anyone in any age is always available on the same principles to anyone willing to abide them at any time. (D&C 130: 20-21.) Do you really believe these teachings of our Lord? Then why not act on them?
I know these things are truly within the reach of almost all of you. The overwhelming majority of readers of this blog have lived better lives than I have You are almost all better qualified than I was. I believed these things, trusted the Lord, acted on His promises. As a result, I am among those who has been "gathered in" and I "know the Lord my God," having been "redeemed by Him." It is more than possible for you.