Paul said that to people who had a faith, but not the mutual faith of both him and them. They had never heard Paul's gospel. The only way to continue in God's goodness would have been for them to be established into the Body of Christ by Paul's gospel...Have you trusted the Lord believing 1 Corinthians 15:1-4 KJV as the ONLY means by which you can be saved? Trust the Lord now!
Paul is speaking to people already grafted in, so they are gentiles who know God. And Paul is also saying that they must remain in God's goodness or they too will be cut off. Even Paul himself believed that he could be cast away if he didn't remain in God's goodness and fight his flesh and keep his body under subjection to do the will of God.
1 Corinthians 9
I therefore so run, not as uncertainly; so fight I, not as one that beateth the air:
But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.
Also, I trust in Jesus Christ and the gospel which he preached, the gospel which the apostle Paul also preached. There is only one gospel of God, not two.
Those that are being saved are doing so by believing in the Lord Jesus Christ and believing in his gospel, through faith by the grace of God.
Also
Romans 1
Paul, a servant of Jesus Christ, called to be an apostle, separated unto THE GOSPEL OF GOD,
2 (Which he had promised afore by his prophets in the holy scriptures,)
3 Concerning his Son Jesus Christ our Lord, which was made of the seed of David according to the flesh;
4 And declared to be the Son of God with power, according to the spirit of holiness, by the resurrection from the dead:
5 By whom we have received grace and apostleship, for obedience to the faith among all nations, for his name:
6 Among whom are ye also the called of Jesus Christ:
7 To all that be in Rome, beloved of God, called to be saints: Grace to you and peace from God our Father, and the Lord Jesus Christ.
8 First, I thank my God through Jesus Christ for you all, that your faith is spoken of throughout the whole world.
9 For God is my witness, whom I serve with my spirit in the gospel of his Son, that without ceasing I make mention of you always in my prayers;
10 Making request, if by any means now at length I might have a prosperous journey by the will of God to come unto you.
11 For I long to see you, that I may impart unto you some spiritual gift, to the end ye may be established;
12 That is, that I may be comforted together with you by the mutual faith both of you and me.
13 Now I would not have you ignorant, brethren, that oftentimes I purposed to come unto you, (but was let hitherto,) that I might have some fruit among you also, even as among other Gentiles.
14 I am debtor both to the Greeks, and to the Barbarians; both to the wise, and to the unwise.
15 So, as much as in me is, I am ready to preach the gospel to you that are at Rome also.
16 For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; TO THE JEW FIRST, AND ALSO TO THE GREEK.
So it's one gospel, and it's the same gospel for the Jews and the gentiles.