"Gospel" means good news — not good advice. Advice tells you what to do; news tells you what has been done. The Christian message is news about what God has done in Jesus Christ.
Paul summarised it as the message "of first importance": "that Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures" (1 Corinthians 15:3–4, ESV).
Notice three things. Christ died — a real death in history. For our sins — he died in our place, carrying what we deserved: "For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God" (2 Corinthians 5:21, ESV). He was raised — the resurrection shows that his sacrifice was accepted and that death itself is defeated.
The gospel is good news for sinners because it is all of grace. We don't make ourselves acceptable to God; God accepts us because of Jesus, and that grace then changes us from the inside out. The best gift of the gospel isn't even forgiveness, wonderful as that is — it is God himself. John Piper's book God Is the Gospel explores exactly this: forgiveness, adoption and eternal life are precious because they bring us home to God.
The gospel isn't only the door into the Christian life; it is the ground we stand on every day. We never move beyond it — we move deeper into it.
Go deeper: R. C. Sproul, What Is the Gospel? (free ebook, Ligonier) · The Gospel Coalition, Foundation Documents.