The gospel is the announcement that Jesus lived, died, and rose to deal with sin completely and reconcile you to God as a gift. It is good news to be believed, not good advice to be performed. Christ did the work; you receive it by faith.
The Grace Answer
The word gospel means good news, and that single fact reshapes everything about it. News is an announcement of something that already happened, not instructions for something you must now pull off. The gospel is not good advice about how to climb to God. It is the report that God came down to you.
Here is the news. Jesus, the Son of God, lived the perfect life you could not live, died the death your sin deserved, and rose again, defeating sin and death. On the cross He dealt with your guilt completely, and in His resurrection He opened the door to new life. And it is not only that a debt was paid on your behalf. In Christ you are brought near, reconciled to God, and joined to Him, so that His life becomes your life. The gospel does not merely improve your record; it makes you new and brings you home. Paul summed it up simply: Christ died for our sins according to the Scriptures, and He was buried, and He rose again the third day.
Received, not achieved
Because it is news of a finished work, the gospel is received by faith, not earned by effort. By grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast. You do not clean yourself up first. You do not contribute your part to complete Christ’s part. You simply trust what He already did and receive what He freely gives.
It is also what separates it from religion. Religion says do, and hope you did enough. The gospel says done, now rest. Every other system in the world spells salvation as a verb you must perform. The gospel spells it as a gift you receive with empty hands. This is why the gospel produces rest rather than anxiety. If salvation depended on your performance, you could never know if you had done enough. Because it depends on His performance, you can know, because He said it is finished. Assurance is built into the good news.
So the gospel is not the first lesson you graduate from into harder religion. It is the whole thing, start to finish. You were saved by grace, you stand by grace, and you will finish by grace, all of it a gift, none of it a wage.