Yeah it was the same Gospel throughout, even what the 12 Apostles preached. The idea of Christ's coming Kingdom is PART of The Gospel of Jesus Christ. And Apostle Paul definitely preached Christ's coming Kingdom also. Who can read Matthew 8 about the centurion (a Gentile), and say our Lord Jesus wasn't speaking about him when saying many will come from the east and west and sit down in His future Kingdom???
The Gospel of Jesus Christ automatically... includes His coming literal Kingdom to be established here on earth, beginning at His return. That is what the symbolic marriage feast in Matthew 8:11 is about that He was pointing to, which is to include the believing Gentiles. There are several Old Testament passages in the prophets that declare this, especially in the Book of Isaiah.
In Acts 10 God showed Apostle Peter that His Salvation was also open to the Gentile, and Peter himself preached Christ crucified to the small group of Gentiles gathered in that chapter.
So did the other Apostles preach the same Gospel of Jesus Christ, and even also to Gentiles! Peter was actually first to preach The Gospel to the Gentile (Acts 10). Men's traditions try to create their own doctrines on all that, but they're just men's leaven doctrines, not actually based on written Scripture.
And let's get it straight per God's Word who all Apostle Paul was 'sent' to preach The Gospel to, as I have already shown but those here on men's false traditions just skate over, which is actual rejection of God's written Word...
Jesus declared this about Apostle Paul's commission in The Gospel...
Acts 9:15
15 But the Lord said unto him, "Go thy way: for he is a chosen vessel unto Me, to bear My name before the Gentiles, and kings, and the children of Israel:
KJV
So if Paul preached a different gospel to Gentiles, then that would mean he preached another... gospel to those kings and to the children of Israel. That thinking of course is men's lie. Paul preached the same Gospel to all three groups.
The gospel of the kingdom was NOT a perversion of The Gospel of Jesus Christ. It's Biblical ignorance to even think such an idea! The ones in Galatians that Paul was against, and warned against that were perverting the one Gospel were Judahizers that crept in and were trying to preach salvation by the law! That was the 'another gospel' that Paul warned the Galations against!
It's amazing how especially, brethren will instead follow a bunch of Pre-trib Rapture scholars into the pit of hell with their false traditions against Christ's future Kingdom to come on earth.
Please allow me to address below this statement first if you will:
"Yeah it was the same Gospel throughout, even what the 12 Apostles preached."
Here are the differences in preaching between the twelve and Paul:
1)The name of Jesus
The twelve preached whosoever believed only in the name of Jesus were saved with no mention of the cross..... John 1:12 John 3:18 John 20:31 Acts 10:43
Paul preached salvation in the name of Jesus and the cross......Galatians 6:14 1 Timothy 1:16
2)Baptism....the twelve preached salvation upon baptism Acts 2:38 Mark 16:16
Paul was not sent to baptize......1 Corinthians 1:17
3)The Holy Ghost.....receiving the Holy Ghost upon baptism.....Acts 2:38
Holy Ghost received upon belief......Ephesians 1:13
4)The kingdom...the twelve preached the kingdom gospel of the earthly Davidic kingdom.... Luke 9:2 Luke 9:6
Paul spoke of a heavenly kingdom.....2 Timothy 4:18
5)Forgiveness...the twelve preached forgiveness at the coming of Christ....Acts 3:19
Paul preached forgiveness now in Christ.....Colossians 1:14
6)Salvation...the twelve preached salvation at the coming of Christ....Acts 3:19 1 Peter 1:5 1 Peter 1:7
Paul preached salvation now.....Ephesians 2:8
7)Destination.....The twelve were inheriting the Davidic kingdom on earth.....Luke 22:30
Paul preached the heavenly places for the body of Christ....Ephesians 2:6
Here are the similarities between the twelve and Paul:
1)The name of Jesus...The twelve preached the name of Jesus....John 1:12
Paul preached the name of Jesus....Acts 9:29 Romans 1:5 1 Corinthians 1:2
2)Jesus the Christ, the Son of God....The twelve preached Jesus as Messiah, the Son of God....Matthew 16:16
Paul preached Jesus as the Christ, the Son of God....2 Corinthians 1:19
3)The gospel of Christ....the twelve preached the gospel of Christ....Mark 1:1
Paul preached the gospel of Christ (but with further revelations)....Romans 1:16
You can decide if they preached the same gospel based on comparing scripture.