Why does Paul use the word "that" when referring to his Gospel?

Les explains that Paul communicated this Gospel—or "that Gospel"—carefully, line upon line and precept upon precept, the Gospel he preached among the Gentiles. The Holy Spirit prompted the word "that" to mark the difference between Paul's Gospel and what Peter, Jesus, and the eleven had been teaching to Israel. Jesus and the twelve operated under the Gospel of the Kingdom: the King is here, ready to set up the Kingdom offered to Israel since Abraham was given the covenant, and that Jesus of Nazareth is the Christ, Israel's Messiah. But the Gospel of Grace is that this same Messiah was crucified, shed His blood, was buried, and rose from the dead as Savior not just of Israel but of the whole human race. These were new revelations not found in the Old Testament, coming after the cross and resurrection were completed.