In brief, the word gospel means Good News.
The good news is that God sent his one and only Son, Jesus Christ, into the world to save sinners.
The Bible tells us plainly that all humanity has sinned against God (Romans 3:9-12, 23 ; 5:12) . This has separated us from God’s favour (Colossians 1:21) and brought us under his just judgment (Hebrews 9:27 ; 2 Corinthians 5:10; 2 Thessalonians 1:5-10 )
However God has shown great mercy in that he sent Jesus Christ into the world to make possible a reconciliation between us and himself.
This is possibly because of Jesus’ life, death and resurrection. Jesus lived a sinless life that fulfilled all of God’s requirements. He then offered up his life on the cross as payment for the sin of all who trust in him as Saviour. In other words Jesus become the believers substitute taking their punishment upon himself. God accepted Jesus sacrifice as fully sufficient to remove the believers guilt and considers them to be covered by the righteousness of Christ, thus they become acceptable to him.
It is when a person confesses before God that they are a sinner and accepts Jesus Christ as their personal Saviour by faith, that they receive forgiveness, eternal life and reconciliation with God. John 3:16; Ephesians 2:8; Romans 10:13; 1 Timothy 1:15; Colossians 1:22
This is only a brief summary, if you would like more information please ask, you can also look at these links.