The other gospels touch on the same theme, but this gospel makes it central. The movement from the kingdom of man to the kingdom of God was illustrated by Matthew's visible transfer of allegiance from Caesar to Christ. A symbol of this transfer of allegiance was Matthew's decision to leave the money table and join the disciples. This means that the kingdom is in history, and it shapes history. It overcomes resistance in history.
Gates are defensive.
Hell has gates. Satan's kingdom is on the defensive today. The gates of hell are historical, part of the historical conflict between good and evil. The conflict is now. The church will not batter down hell's gates in the nether world.