Albert Barnes on Mark 16:15

Into all the world. To the Gentiles as well as the Jews. This was contrary to the opinions of the Jews, that the Gentiles should be admitted to the privileges of the Messiah's kingdom, or that the partition wall between them should be broken down. See Acts 22:21,22. It was long before the disciples could be trained to the belief that the gospel was to be preached to all men; and it was only by special revelation, even after this command, that Peter preached to the Gentile centurion, Acts 10:1. Jesus has graciously ordered that the preaching of the gospel shall be stopped by no barriers. Wherever there is man, there it is to be proclaimed. To every sinner he offers life; and all the world is included in the message of mercy, and every child of Adam offered eternal salvation.

Preach. Proclaim: make known: offer. To do this to every creature, is to offer pardon and eternal life to him on the terms of the plan of mercy--through repentance and faith in the Lord Jesus.

The Gospel. The good news. The tidings of salvation. The assurance that the Messiah is come, and that sin may be forgiven, and the soul saved.

To every creature. To the whole creation. That is, to every human being. Man has no right to limit this offer to any class of men. God commands his servants to offer the salvation to all men. If they reject it, it is at their peril. God is not to blame if they do not choose to be saved. His mercy is manifest; his grace is boundless in offering life to a creature so guilty as man.

(b) "creature" Rom 10:18, Col 1:23