NOTE: I do want to qualify that the word “gospel” is used in different ways in Scripture—
First, “gospel” can be used in a general sense—describing all the doctrines of Christ and His apostles. And in this sense, it comprehends the law as well as the gospel.
Second, “gospel” is used in the strict or proper sense, i.e. “good news, glad tidings, or a joyful message.” And it refers to the free grace of the gospel apart from our works. It’s this second sense with which I’m most concerned—the proper use of the term.
The gospel, in the strict sense, is what our triune God does for us. Full stop.
We should be careful not to insert ourselves, or what we do into the gospel equation. If we do this, we essentially become co-mediators and co-redeemers with Christ. Let’s take our cue from Mark 1:15 and other illuminating texts—
In places like Mark 1:15, it becomes very clear that the “gospel” is something distinguishable from commandments requiring us to do something, e.g. “Repent, and believe…” And while the ability to repent and believe is given in the gospel, our actual repenting and believing are not the gospel but our obedience to God’s commands. Our text presupposes this. The gospel is that in which we believe. Our belief is not itself the gospel. Our repentance is not itself the gospel. So what is the gospel?
First, according to our text, the gospel is something we “believe in,” it is something to be believed. More technically, it is something to be apprehended by faith. We might even say that the gospel is something in which rest instead of something for which we work. And this is an incredibly helpful starting point for understanding what the gospel is fundamentally.
If the gospel is something we are to believe, something that is given by God and apprehended only by faith, then it is distinguished from the works of the law. In Romans 4:5, Paul makes this distinction, “But to him who does not work but believes on Him who justifies the ungodly, his faith is accounted for righteousness…” The gospel is that in which we believe. It is not that for which we work. And so, the first observation in terms of what the gospel is is that it is to be distinguished from what we do. And this means it comes to us freely since it’s not conditioned upon anything in us or anything from us.
Second, the gospel is good news. What is the good news? Paul, in Romans 1:16, says, “For I am not ashamed of the gospel of Christ, for it is the power of God to salvation for everyone who believes, for the Jew first and also for the Greek.” The gospel is the “good news of Christ…” So, it comes through the Son of God incarnate, the Messiah, the Lord Jesus Christ. And in terms of what it is, it is the “power of God unto salvation…” It’s not the power of man. It’s not the works of man. It’s not man’s act or quality of repentance, his act or quality of faith—though both of these things result from the power of God unto salvation. The gospel is the power of God unto salvation “for everyone who believes, for the Jew first and also for the Greek.”
Third, this power of God unto salvation which comes through Jesus Christ (alone) refracts into specific aspects—the Person of Christ sent, the work of Christ accomplished, the work of Christ applied to the believer by the Holy Spirit, and the hope of glory at the end. All of these are expressions of God’s power worked out by God Himself in favor of our salvation. But then this power, accomplished in history, is applied to us in terms of justification, adoption, sanctification, and (eventual) glorification. These are things that happen to us, not things we cooperate with. Justification is freely given. Adoption is freely given. Sanctification is freely given. Glorification is freely given. You get the idea.
John Colquhoun (18th -19th c.) helpfully defines the gospel in the following way—
The gospel reveals to us what the Father, the Son, and the Holy Spirit have done for us and are willing to impart to us, how fully and freely these are offered to us, and how they are to be received and enjoyed as gifts of infinitely free and sovereign grace.
In summary, when we speak of the gospel in the most technical sense, it refers to what God has done for us through Christ. The good news is what God does for us, not what we do for God. Understanding this helps us to understand that our redemption, from beginning to end, is of God. The reason why we are made right before God—why God is pleased with us—is not to be found in ourselves but solely in what God has done for us through Christ. And when we understand this, then and only then will our assurance be properly grounded.
 John Colquhoun, A Treatise on the Law and the Gospel, (Grand Rapids, MI: Reformation Heritage Books, 2022), 102.