What is the Gospel? Why does it seem to change as we read the Bible?

The gospel is simple

G: God Created the world good (Rev 4:11)
O: man Opposed God, and rejected him, wanting to run his own life (Rom 3:10-12)
S: Man is Sinful God is Holy, man is Sentenced to death (Rom 6:23)
P: The Person of Jesus Christ entered the world to solve the problem of Sin (1 Pet 3:18)
E: After his death, Jesus' tomb was Emptied, and he was raised from the dead ( 1 Pet 1:3)
L: We all must a choice to Live our way or God's way (John 3:36)

In other words, GOSPEL means Good news, this Good news is that God has made a way for us to be accepted by him, even though he is perfect and we are sinners.

Gospel for Abraham - 'in you shall all the nations be blessed'

Galatians 3:8

(3:8) And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, “In you shall all the nations be blessed.”

 According to this verse, the gospel was even preached to Abraham. Did it look like it does today. "God loves you, he sent Jesus to die for your sins, so now if you believe in him you will have eternal life?"
Abraham did not even know Jesus, and Jesus had not even died yet, so how could the gospel in our general understanding of it be communicated to Abraham?

This is the passage referred to:

Genesis 12:3

(12:3) I will bless those who bless you, and him who dishonors you I will curse, and in you all the families of the earth shall be blessed.” 

From this verse I gather that through Abraham blessing will be available to all people. This blessing will come through Abraham.

In Genesis 22:18 we get additional information, "in your offspring"

(22:18) and in your offspring shall all the nations of the earth be blessed, because you have obeyed my voice.”

This verse is interpreted for us in our God inspired commentary - The Bible! - Book of Acts.
We see how all prophecies come together in Jesus, even the promise made to Abraham!

Acts 3:19-26 - 

(3:19-26)  19 Repent therefore, and turn back, that your sins may be blotted out, 20 that times of refreshing may come from the presence of the Lord, and that he may send the Christ appointed for you, Jesus, 21 whom heaven must receive until the time for restoring all the things about which God spoke by the mouth of his holy prophets long ago. 22 Moses said, ‘The Lord God will raise up for you a prophet like me from your brothers. You shall listen to him in whatever he tells you. 23And it shall be that every soul who does not listen to that prophet shall be destroyed from the people.’ 24And all the prophets who have spoken, from Samuel and those who came after him, also proclaimed these days. 25You are the sons of the prophets and of the covenant that God made with your fathers, saying to Abraham, ‘And in your offspring shall all the families of the earth be blessed.’ 26God, having raised up his servant, sent him to you first, to bless you by turning every one of you from your wickedness.”

This passage is great: First it explains how the promise to Abraham (all the families of the earth would be blessed through his offspring), is fulfilled in Jesus Christ.

If we look at the Bible as a whole - everything started well in Genesis 1&2. Everything changed with the fall (Genesis 3). Genesis 8:21 tells us that right from our youth our heart is bent towards evil! We can deny it all we want, but mankind is pretty messed up (just turn on the News!).
God has been actively working throughout history to restore mankind that has been separated from Him due to their Sin (Isaiah 59:2).

Throughout the Old Testament, according to this passage in Acts (quoted above) the prophets tell us of a time in the future where God would bless all the Nations. Early on we knew this would happen through the descendants of Abraham (Gen 22:18) as time moved on, we are told that he would be one from the line of King David (2 Sam 7:12-14). This passage in Acts confirms that this has now happened through Jesus.

Gospel of Jesus - 'Repent for the Kingdom of heaven is at hand'

The Gospel preached to Abraham was that through his descendants all the families of the world would be blessed. Additional information is given to King David (a descendant of Abraham) that the one through whom all will be blessed will come through his line.

After David, it gets rocky for Israel, they get invaded by the Assyrians, then the Babylonians, then the Persians, then the Greeks, then the Romans (New Testament period)...

The Jews know God's promises and they are waiting for that King from the Line of David (part of the offspring of Abraham) to come. (Seen in Acts 1:6)

Jesus comes around proclaiming the Gospel of the Kingdom (Mat 4:23). Previously in Matthew 4:17 more detail is given: "Jesus began to preach, saying: 'Repent for the kingdom of heaven is at hand'.

How do the gospels of Abraham and Jesus fit together?

Abraham was promised his offspring would bless everyone. In the context of the Bible, the Righteousness of God, the sinfulness of Man, could this mean that God would restore humanity from the consequences of the Fall?

King David is promised an heir who will rule forever. The prophets keep talking about the coming of a Messiah King. 

Christ's good news (Gospel), is that the fulfillment of all these things, the Blessings of Abraham, and the Messiah King, had now come near (him being the King). What you need to be part of this kingdom is  to admit that you had broken God's Law, and that you needed to Repent! (Matthew 4:17).

The Gospel after Jesus Death? - 'death, burial and resurrection of Christ'

John the Baptized said the following when he saw Jesus: 

John 1:29 

“Behold, the Lamb of God, who takes away the sin of the world!

The Acts 3:19 passage clearly says that for our sins to be forgiven we must repent!

Colossians

(1:20-22)  20and through him to reconcile to himself all things, whether on earth or in heaven, making peace by the blood of his cross.  21And you, who once were alienated and hostile in mind, doing evil deeds, 22he has now reconciled in his body of flesh by his death, in order to present you holy and blameless and above reproach before him,

 Through the Blood of Christ, though we were practicing evil things, God has reconciled us in Christ, that we might be presented as holy before him.

So when Paul mentions clearly what the gospel is: "Christ died, was buried and was raised" it also then fits perfectly with the others "gospels".

1 Corinthians 15:1-4

(15:1-4)  1Now I would remind you, brothers, of the gospel I preached to you, which you received, in which you stand, 2and by which you are being saved, if you hold fast to the word I preached to you— unless you believed in vain.  3For I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, 4that he was buried, that he was raised on the third day in accordance with the Scriptures,

 Conclusions:

This is the gospel:

• According to Galatians 3:8, Abraham was given the gospel in Genesis 12:3 - I will bless those who bless you, and him who dishonors you I will curse, and in you all the families of the earth shall be blessed.” 
• When Jesus comes it sounds more like this: Mat 4:17 “Repent, for the kingdom of heaven is at hand.”
• Paul makes it very clear that the Gospel is the Death, burial and Resurrection of Jesus Christ (1 Cor 15:3-4)

So how do they all fit together?

Due to the Fall, our sins separate us from God (Is 59:2). God is actively finding a way to reconcile the world, and promises Abraham this will happen through his offspring. Do go from the blessing of Abraham to the Kingdom of Jesus. God tells David that Abraham's defendant will come from his line of Kings.

Christ proclaims the Good news that he is this King of the line of David. In order for the people to receive the blessing of being reconciled to God, to have eternal live they must believe in Him (John 3:16). Through his sacrifice on the Cross, all those who have accepted Christ can now be seen as Holy before God ( John 1:12, Col 1:22, 2 Cor 5:21). All this is possible through Jesus' death, burial, and resurrection. (1 Cor 15:3-4). He died because a sacrifice was necessary for the forgiveness of sins (Hebrew 9:22), his burial showed to everyone, that he was truly dead. His resurrection confirmed that Jesus Christ was the one he claimed to be, that God does have the power to rise people from the dead, and this give us hope also that when Christ returns if we have died already we too will be raised from the dead (1 Cor 15:20-22).