I believe that the 144,000 are actually Jewish people because at that time the Jews will recognize Jesus as their Savior.
God said He is going to make a full end of all Gentile nations but He will not make a full end of Israel although they will not be wholly unpunished.
God said He is going to gather all nations together to battle against Israel,all the Gentiles that do not love God,and then when they attack Israel God will put them down,and Israel will be the only operating nation in the millennial reign of Christ.
So it would seem at that time that they would be actual physical Jews.
The tribe of Dan was not included but Mannaseh was included,which could be because there is not enough of the tribe of Dan,or because Dan introduced Israel to idolatry.