Must first be preached ( prōton dei kēruchthēnai ).

This only in Mark. It is interesting to note that Paul in Col 1:6, Col 1:23 claims that the gospel has spread all over the world. All this was before the destruction of Jerusalem.

Mat 24:14.

"for a witness, and then shall the end come" (Mat 24:14). God never sends judgment without previous warning; and there can be no doubt that the Jews, already dispersed over most known countries, had nearly all heard the Gospel "as a witness," before the end of the Jewish state. The same principle was repeated and will repeat itself to "the end."

And the Gospel must first be published among all nations. - Many of the Evangelistaria omit this verse. Its proper place seems to be after verse the thirteenth.

Mar 16:15; Mat 24:14, Mat 28:18, Mat 28:19; Rom 1:8, Rom 10:18, Rom 15:19; Col 1:6, Col 1:23; Rev 14:6

I am prone to think that our Lord gives this not only as a sign of the destruction of Jerusalem, but of the end of the world, and the latter principally; for before the destruction of Jerusalem (which was in less than forty years after Christ’ s death) the gospel was not preached to all nations, otherwise than as all signifies very many. And I do think that all places shall have the gospel preached to them before the day of judgment, after another manner than either it was possible it should be preached to them within forty years after the death of Christ, or than many places have had it preached amongst them to this day. For though the Holy Scriptures, and ecclesiastical historians, give us a somewhat large account of the gospel being preached in Europe, Asia, and in Africa, yet we have little account from any of them of its being preached in America. I am not wholly ignorant of what those writers tell us, of Thomas the apostle’ s preaching to the Indians, and of Trumentius and his colleague, but there are very few preachers that any stories give an account of gone to the Indians, whither I believe the gospel must go before that Christ comes to judgment.

And the Gospel must first be published among all nations. The Syriac version reads, "my Gospel"; the Gospel which Christ was the author, subject, and preacher of; this "must be published". There was a necessity of the promulgation of it by the will of God, the command and commission of Christ; and for the gathering in of the Jews, that were the elect of God, "among all nations" of the world, especially in the Roman empire; and that "first", or before the destruction of Jerusalem; See Gill on Mat 24:14.