The Gospel of Jesus Christ is: That God has sent His only Son, Jesus, to earth to preach the good news and to die on the cross for our sins which separate us from God. God, our heavenly Father, then raised Jesus from the dead to be in heaven. By doing this He provided the way for us to return to proper relationship to God.
Becoming a "born again" Christian then, is trusting (having faith in) Jesus' work on the cross as the complete payment for our sins. This puts us in right standings (righteous) with God. His resurrection is proof of God's work in the process. It also was necessary for Jesus to send the Holy Spirit to fill us as believers. The Spirit continues Jesus' ministry in and through us, teaching us, leading us, and changing us into children who are godly in action as well as by faith.
If Christianity is new to you the following terms may help:
The Gospel was the good news first for the Jews. The message two thousand years ago and still today is that Jesus is the Messiah, the anointed messenger of God, and most importantly, He is the "Lamb of God" (signifying His sacrificial ministry). The Gospel came through the Jews. In fact, Jesus was Jewish and was legally a descendent of King David.
To see how The Lord Jesus fulfills prophecy in the Hebrew Bible go to The Gospel of Matthew. Matthew was a Jew that lived almost 2000 years ago. He became one of Jesus' main disciples and latter in life wrote his account of the life, death, and resurrection of Jesus. His account seems to have been written with a Jewish audience in mind.