The gospel is the good news that God's kingdom has come to earth. This means God's Love & justice for all men has come. The good news is that through His kingdom He has a plan to put an end to all the evil in this world and the death it causes.
We are invited to believe this good news and turn from our own evil. God sent Jesus, His Son, to bring His Kingdom and rule as King over all things. He sent Jesus to first live righteously and then to die in our place to pay for our evil. Through Jesus' death He defeated evil and death and then God raised Him up from the dead to rule over Heaven and Earth.
Through Jesus' death we can be forgiven of our evil so we do not fall under God's just judgement of evil. We now can live under Jesus as King and follow His good commandments until He returns. When Jesus returns He will judge all mankind for our good and evil and whether we accepted and knew Him as our Savior and King. If we believe this good news we will become righteous rulers with Him forever.