The Gentiles are grafted in, however, they are not Gentiles in this dispensation, the body of Christ, they are members of the kingdom saints, the remnant of Israel who will inherit the kingdom on earth:
Romans 9 through 11 is about Israel, not the church, the body of Christ.
Romans 9:24 Not only the Jews believed the gospel of the kingdom, and the preaching of Jesus and the twelve, but also Gentiles (Acts 10:45). This was not our gospel of grace. They only believed on the name of Jesus (John 3:18 John 20:31).
Romans 9:27 This is identifying the remnant of Israel as what Paul is talking about. The remnant is the believing Jews and Gentiles who will enter the promised kingdom on earth (Isaiah 2:2-3 Exodus 19:5-6).
Romans 9:30 Those Gentiles had faith, which Israel rejected.
Romans 11:5 At the time Paul was writing, the remnant of Israel, Jew and Gentile, were participants in the church.
Romans 11:11 The believing Gentiles would provoke unbelieving Israel to jealousy.
Romans 11:13 Paul was speaking to the Gentiles, not in the body of Christ, but in the remnant of Israel.
Romans 11:17 The Gentiles were grafted into the remnant of Israel.
Romans 11:20 Unbelieving Israel was broken off the tree. The Gentiles were grafted in by faith
Romans 11:20 The Gentiles could also be broken off branches.
Romans 11:25-27 Israel will be saved.
On the weekend of Jesus' arrest, trial, and crucifixion He opened the future to His disciples in Matthew 24. In that passage He tells them of the destruction of Jerusalem and the signs of the end of time. In that passage He says the following:
Matthew 24:14 And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.
Now just what gospel was Jesus referring to when He says "this gospel"? Is He not referring to what He has been teaching during His entire ministry? How can He be referring to anything that was supposedly to come afterward? His usage of the word "this" eliminates that possibility for His disciples knew of no other gospel than that which Jesus taught every day. Jesus is telling His disciples that what He has been teaching them will be taught to all nations, meaning the Gentiles i.e. non-Jews, and then the end would come.
Jesus is quite plainly contradicting your assertions. And the only conclusion that can be drawn from His statement is that the gospel to the Gentiles was going to be the same gospel that He was teaching to the Jews. There is no other way to read His very plain words.