Greetings again “heir”, The Apostles were commanded to go into all the world and preach the gospel the gospel to every creature, thus teaching all nations, not just to the Jews:
As with everything with God, there was an order for them. The apostles were under this command of the Lord:
Matthew 10:5 These twelve Jesus sent forth, and commanded them, saying, Go not into the way of the Gentiles, and into any city of the Samaritans enter ye not:
Matthew 10:6 But go rather to the lost sheep of the house of Israel.
until
Matthew 10:23 But when they persecute you in this city, flee ye into another: for verily I say unto you, Ye shall not have gone over the cities of Israel, till the Son of man be come.
The "go ye therefore", when they will go and teach all nations is when the Lord returns (second coming) with "all power"...
As to now (nothing to do with what was and will go on in Matthew, Mark, Luke or John) where the righteousness of God without the law is manifested, Paul is the apostle of the Gentiles (Romans 11:13 KJV) given the dispensation of the grace of God to us (Ephesians 3:1-9 KJV). We are partakers by the gospel (1 Corinthians 15:1-4 KJV, Ephesians 3:6 KJV) and our only baptism is identification BY ONE SPIRIT IN ONE BODY (1 Corinthians 12:13 KJV). Where we are concerned, the Spirit is the baptizer, not Jesus "with the Holy Ghost", by man (like John the Baptist or Peter) in water (a baptism of repentance for the remission of sins), or fire as in other places in the Bible. Our baptism is unique to just the church, which is His Body!
The Apostles were to preach the gospel and there is no indication that there were two different gospels:...
You keep saying "the apostles" as if every time any apostle in the Bible is mentioned or separated unto a gospel, it includes all of them. What does verse 1 of Romans 1 say? Does it say "the apostles" or does it say "Paul, a servant of Jesus Christ, called to be an apostle, separated unto the gospel of God,"? Why "the gospel" talk as if there is only one? Since you cited Romans, I'll use them to demonstrate more than one gospel.
The Romans had a faith in the gospel of God/the WHO of Jesus Christ (Romans 1:1-4 KJV that's one gospel) but needed to be established by Paul's gospel/the gospel of Christ/the WHY of the cross. It's the purpose of Paul's letter to them (Romans 1:9-15 KJV)! Paul took the first 8 chapters of Romans to describe it. To summarize the WHY of the cross in one verse in Romans, I would send you to Romans 4:25 KJV. That gospel (Paul's gospel) was before a mystery kept secret until revealed to and through and committed to Paul by God only be wise (Romans 16:25-27 KJV, 1 Corinthians 2:6-8 KJV, Galatians 1:11-12 KJV). None of the above in Romans, (with the exception of the gospel of God) has to do with the gospel of the kingdom in time past or ages to come in Matthew, Mark, Luke, John, Hebrews through Revelation when it will be preached again (not to mention the everlasting gospel preached by angels in Revelation). You need to rightly divide the gospel of YOUR salvation with those. What is the gospel of YOUR salvation?
Galatians 3:16, 26-29 identifies the Gentiles of Galatia as the inheritors of the Abrahamic promise concerning the land Genesis 13:14-15.
No, the covenant of promise to which the Gentiles to whom Paul was first sent (including the Galatians) were benefactors is a blessing for blessing Israel according to Genesis 12 and the promise of the Spirit through faith.
Genesis 12:1 Now the LORD had said unto Abram, Get thee out of thy country, and from thy kindred, and from thy father's house, unto a land that I will show thee:
Genesis 12:2 And I will make of thee a great nation, and I will bless thee, and make thy name great; and thou shalt be a blessing:
Genesis 12:3 And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed.
...
Galatians 3:14 That the blessing of Abraham might come on the Gentiles through Jesus Christ; that we might receive the promise of the Spirit through faith.
They had heard Paul preach the one Gospel, and they had then been baptised into Christ by water baptism.
"one gospel" is a made up term and it won't fly here. The Galatians were not baptized "by water" as you say, but identified Into Christ's death (Romans 6:3-4 KJV). That only happened for them and us by extension "by one Spirit" (1 Corinthians 12:13 KJV), not by water!
There are not true gospels:
Galatians 1:6–9 (KJV): 6 I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel: 7 Which is not another; but there be some that trouble you, and would pervert the gospel of Christ. 8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. 9 As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed.
In the passage you cited, there are no false gospels only very real gospels that were being preached. "Unto you" the Galatians, had any of those other gospels been preached to them as unto them, it would have been a perversion of the gospel of Christ which is exactly what the Galatians were being bewitched to believe as they were being constrained to be circumcised and keep the law of Moses (Acts 15:5 KJV) all in keeping with other gospels in the Bible but not Paul's gospel).
All of the Bible does not say all the same thing to the same people anymore than we are all going to the same place. You must 2 Timothy 2:15 KJV!