In short the gospel is this:
All men have sinned and come short of the glory of God. Not any one man is good, the only One being good, is God. We have no ability to be God, and therefore non of us deserve heaven and eternal life. For us, this means that the only final destination when we die is hell. All of us will die one day, and after this the judgment. We will all fail the test.
Now because of this, in God’s great mercy, he sent his son Jesus Christ to earth to die for our sins. To take upon himself the atrocity of our failing, and sacrifice his life in our stead. Christ was the only one able to do this, and to rise up from the dead, victorious over death.
The gospel of Jesus Christ is simple: believe on him and repent. Ask God to break your heart over your own sin, understand that you fall short of God’s glory and deserve death. Than, and only then will you have a contrite heart, able to receive the free eternal gift of salvation.