Niels Hemmingsen likes to define things. Herewith his definition of the Gospel from his Enchiridion Theologicum.
The Gospel is a doctrine revealed by God, in which liberation from the curse of the Law and the wrath of God is announced and remission of sins, salvation, and eternal life to those who believe in the Son is proclaimed, on account of the sacrificial offering [victimam] of the same, according to the promise made long ago to the Fathers; in order that the glory of the goodness of God may always be extolled and, furthermore, that men freed by Christ may bring forth fruits worthy of the Gospel, and may, in the end, enjoy eternal life. 1
For Hemmingsen, the Gospel is a message–an announcement, a proclamation–of freedom [liberatio] and forgiveness [remissio] for those who believe in the Son, secured by the Son, in keeping with God’s word of promise. It has three purposes [ut] in this definition: the public display and praise of the glory of God; the bringing forth of fruits in the lives of those who have been freed; and the enjoyment of everlasting life. There is something interesting in these last two purposes, both of which relate directly to man. The Gospel, which announces man’s free justification in the remission of his sins and liberation from the curse of the Law, is ordered toward his sanctification and glorification as well. These things are not the “Gospel” itself, but they fall within the intentions of its proclamation.