The Gospel is "good news". The good news is the good news of the Kingdom of God:
Mark 1:14-15 KJV
Now after that John was put in prison, Jesus came into Galilee, preaching the gospel of the kingdom of God, [15] And saying, The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel.
John the Baptist preached the same thing:
Matthew 3:1-2 KJV
In those days came John the Baptist, preaching in the wilderness of Judaea, [2] And saying, Repent ye: for the kingdom of heaven is at hand.
The disciples preached it (before Jesus died):
Luke 9:1-2 KJV
Then he called his twelve disciples together, and gave them power and authority over all devils, and to cure diseases. [2] And he sent them to preach the kingdom of God, and to heal the sick.
Luke 9:6 KJV
And they departed, and went through the towns, preaching the gospel, and healing every where.
The Gospel was proclaimed or preached to Abraham:
Galatians 3:8 KJV
And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying , In thee shall all nations be blessed.
And in that we see the Gospel message was that in Abraham all nations would be blessed.
The Gospel was preached to the Exodus generation:
Hebrews 4:2 KJV
For unto us was the gospel preached,
as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it .
The message given to the Exodus generation was the entirety of the covenant revelation, including not only how they were to live (the commands of God) but also the promises, especially the promises of being the people of God and all that such a relationship included.
The Gospel has always been around, it was there in Abraham's day, the Exodus, John the Baptist's day, during Jesus' earthly ministry, and ever since. It is not JUST the death, burial, and resurrection of Christ (is that what was preached to Abraham?), nor is it just the promise of eternal life in resurrection. It includes the whole entirety of Christian doctrine:
Mark 1:1 KJV
The beginning of the gospel of Jesus Christ, the Son of God;
2 Corinthians 9:12-13 KJV
For the administration of this service not only supplieth the want of the saints, but is abundant also by many thanksgivings unto God; [13] Whiles by the experiment of this ministration they glorify God for your professed subjection unto the gospel of Christ, and for your liberal distribution unto them, and unto all men ;
It is a term that sums up everything God wants you to hear, know, believe, obey, and support, be involved in, promote, and live by: THE GOOD NEWS sent by God.