The English word "gospel" comes from the Anglo-Saxon "God-spell" which means "God-story." "Gospel" translates the Greek word euangelion which occurs more than seventy-five times in the New Testament. Its basic meaning is "good news."
Matthew and Mark primarily use "gospel" in reference to the kingdom in phrases like "the gospel of the kingdom" or "the gospel of God" which was being preached by Jesus (Mt. 4:23; 9:35; Mk. 1:14-15). The story Mark told of Jesus was the "gospel" (Mk. 1:1), so in the second century the first four documents of the New Testamnt came to be called Gospels.
Paul is the New Testament writer who most often uses the term "gospel." Paul was set apart as an apostle in order to preach the gospel to a lost and dying world (Rom. 1:1; Eph. 3:7; Gal. 2:7; 1 Cor. 9:16, 22-23). If the gospel was not believed and obeyed, its hearers would be lost (2 Thes. 1:9). If it was received in faith, it became the instrument for one's salvation (Rom. 1:16).
What is the content of the "gospel" or this good news? The gospel tells of the sacrificial death and resurrection of Jesus Christ (1 Cor. 15:1-4). It is the "gospel of God" because it tells of God's redemptive work (Rom. 1:1; 15:16; 2 Cor. 11:7; 1 Thes. 2:2, 8-9). Since this redemptive work was accomplished in Christ, it is called the "gospel of Christ" (Rom. 1:9; 1 Cor. 9:12; 2 Cor. 2:12; 9:13; 10:14; Gal. 1:7; 1 Thes. 1:8; 3:2). Because of what it promises, it is the "gospel of salvation" (Eph. 1:13) or the "gospel of peace" (Eph. 6:15).
What the gospel entails is summarized by C. H. Dodd from an analysis of early Christian preaching in the book of Acts: (1) the age of fulfilment has dawned, the "latter days" foretold by the prophets (2:16; 3:18, 24); (2) this has taken place through the ministry, death, and resurrection of Jesus Christ; (3) by virtue of the resurrection Jesus has been exalted at the right hand of God as Messianic head of the new Israel (Acts 2:33-36; 4:11; 5:31); (4) the Holy Spirit in the Church is the sign of Christ's present power and glory (2:17-21, 33; 5:32); (5) the Messianic Age will reach its consummation in the return of Christ (3:20; 10:42); (6) the preaching of the gospel closes with an appeal for repentance, the offer of forgivness and of the Holy Spirit, and the promise of salvation (2:38; 3:19, 25; 4:12; 5:31; 10:43).