The word gospel, most of the time, is derived in the KJV New Testament from the Greek euaggelion (Strong's Concordance #G2098). Found 104 times in 98 verses, it means "a good message" or "good tidings." Another word, euaggelizo (Strong's #G2097), is translated as the phrase "preached the gospel" or variation thereof in additional verses such as Luke 4:18, 7:22, 9:6, 20:1 and so on.
The Bible also appends various phrases immediately after "gospel" such as, "of the kingdom" (Matthew 4:23), "of Jesus Christ" (Mark 1:1), "of the Kingdom of God" (Mark 1:14), "of the grace of God" (Acts 20:24), "of peace" (Romans 10:15) and others.
An Expanding Message
The earliest chronological mention of the good news of the Kingdom occurs when Jesus' public ministry kicks off after the arrest of John the Baptist.
Now after the imprisonment of John, Jesus came into Galilee, proclaiming the gospel of the kingdom of God and saying, "The time has been fulfilled, and the kingdom of God is near at hand; repent, and believe in the gospel" (Mark 1:14 - 15, HBFV).
Note that the Lord's message does not mention his death or resurrection as it has yet to take place. Instead, he preached the good news that God's Kingdom would soon come to the earth! He also proclaimed that entrance into this glorious Kingdom as God's spiritual sons and daughters was through repentance and believing the Father's words.
The depth and breathe of the gospel was greatly expanded in the Great Commission given by Christ to his disciples on his ascension day. The disciples were still required to announce the coming of the Kingdom as Jesus proclaimed. Added to it, however, were the Lord's teachings given during his ministry which he wanted conveyed to the world (Matthew 28:19 - 20).
The last expansion of what was considered the gospel came soon after the Lord's departure. Starting on Pentecost in 30 A.D., it would also include what Jesus did (his death, resurrection, etc.), is doing, and will do to insure God's plan that eventually all humanity has a full chance to enter the Kingdom is fulfilled (see Acts 2, 1Corinthians 15:1 - 4).
Counterfeits
The Galatian churches (Iconium, Lystra, Derbe, Pisidian Antioch) were started and nurtured by the Apostle Paul during his first and second missionary journeys. In his absence, however, lies and deceptions quickly moved in which were adopted by the members. The Galatian's breath-taking abandonment of God's truth astonished Paul and motivated him to write the strongest correction letter of his ministry.
I am astonished that you are so quickly being turned away from Him Who called you into the grace of Christ, to a different gospel . . . but there are some who are troubling you and are desiring to pervert the gospel of Christ (Galatians 1:6 - 8, HBFV).
Paul's concern and frustration led to asking, twice, that those who are deceiving the churches be cursed (Galatians 1:8 - 9)! Those promoting such lies were likely false apostles (2Corinthians 11:13) or false prophets and teachers (2Peter 2:1, 1John 4:1). Their false gospel centered on earning God's justification and salvation through works like circumcision (Galatians 2:16, 3:1 - 5, 10 - 13, 5:1 - 4, 6:12).