The word “gospel” means simply “good news”. Scripture teaches that there is an inexhaustible amount of good news available to believers because of Jesus Christ. Paul calls them the “unsearchable riches of Christ” (Eph 3:8). Some examples of these include:

• We are united as one Spirit with Christ (1 Cor 6:17)
• Christ lives in us as the hope and seal of the glory that we are to receive when we see him face to face (Col 1:27)
• We have been freed from the power of sin and no longer slaves to it (Romans 6:6-7)
• We will receive a new resurrection body when Christ returns (1 John:2-3, 1 Cor 15, et al)

Many of these great, unsearchable riches of Christ become more apparent to us as we mature, study, and are discipled into the teachings and knowledge of Christ through scripture. Some we will never fully understand or grasp until we can see Christ face to face and fully know as we are known (1 Cor 13:12). Even if we desire to know and understand all the deeper truths of God, we cannot do this until He permits us to go onto maturity according to His timing and perfect will (Hebrews 6:1-3). These topics are good and should be taught as part of discipleship in the church.

With that said, the “Gospel” that the apostles preached after Pentecost was a specific message outlining Christ’s death, burial, and resurrection (1 Cor 15:1-4). What was the good news about that? Specifically that we can be saved. Our sins would be forgiven, Christ’s righteousness would be imputed to our account, and we would receive His eternal Life and Holy Spirit instead of perishing in our former sin and separation from God.

Here are a few verses showing how the apostolic Gospel was specifically a message of salvation:

Mark 16:15-16
And He said to them, “Go into all the world and preach the gospel to all creation. He who has believed and has been baptized shall be saved; but he who has disbelieved shall be condemned.

Acts 13:26
“Brethren, sons of Abraham’s family, and those among you who fear God, to us the message of this salvation has been sent.

Acts 13:46-49
Paul and Barnabas spoke out boldly and said, “It was necessary that the word of God be spoken to you first; since you repudiate it and judge yourselves unworthy of eternal life, behold, we are turning to the Gentiles. For so the Lord has commanded us,
‘I have placed You as A light for the Gentiles,
That You may bring salvation to the end of the earth.’”
When the Gentiles heard this, they began rejoicing and glorifying the word of the Lord; and as many as had been appointed to eternal life believed. And the word of the Lord was being spread through the whole region.

After there had been much debate, Peter stood up and said to them, “Brethren, you know that in the early days God made a choice among you, that by my mouth the Gentiles would hear the word of the gospel and believe. And God, who knows the heart, testified to them giving them the Holy Spirit, just as He also did to us; and He made no distinction between us and them, cleansing their hearts by faith. – Acts 15:7-9

Acts 16:17
Following after Paul and us, she kept crying out, saying, “These men are bond-servants of the Most High God, who are proclaiming to you the way of salvation.”

Acts 28:28-29
Therefore let it be known to you that this salvation of God has been sent to the Gentiles; they will also listen.” [When he had spoken these words, the Jews departed, having a great dispute among themselves.]

1 Corinthians 15:1-2
Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand, by which also you are saved, if you hold fast the word which I preached to you, unless you believed in vain.

Romans 1:16-17
For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek. For in it the righteousness of God is revealed from faith to faith; as it is written, “ But the righteous man shall live by faith.”

Ephesians 1:13-14
In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise, who is given as a pledge of our inheritance, with a view to the redemption of God’s own possession, to the praise of His glory.

Hebrews 2:3-4
how will we escape if we neglect so great a salvation? After it was at the first spoken through the Lord, it was confirmed to us by those who heard, God also testifying with them, both by signs and wonders and by various miracles and by gifts of the Holy Spirit according to His own will.

1 Peter 1:9-12
obtaining as the outcome of your faith the salvation of your souls.
As to this salvation, the prophets who prophesied of the grace that would come to you made careful searches and inquiries, seeking to know what person or time the Spirit of Christ within them was indicating as He predicted the sufferings of Christ and the glories to follow. It was revealed to them that they were not serving themselves, but you, in these things which now have been announced to you through those who preached the gospel to you by the Holy Spirit sent from heaven—things into which angels long to look.

Acts 11:13-14
And he reported to us how he had seen the angel standing in his house, and saying, ‘Send to Joppa and have Simon, who is also called Peter, brought here; and he will speak words to you by which you will be saved, you and all your household.’