Besides the Pauline epistles, I can't find anything that refers to the salvation of the gentiles, which makes me doubt if it is real or Paul is an impostor. Can you show me some proof that salvation is for all apart from the Pauline letters?
The Gospel of John makes clear salvation is for all people:
And as Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up, that whoever believes in him may have eternal life. (John 3:14-14 ESV)
John is also purposeful to include events such as the encounter with the Samaritan women to show the Gospel was intended for all people.
The public ministry ends when Greeks ask to see Jesus:
Now among those who went up to worship at the feast were some Greeks. So these came to Philip, who was from Bethsaida in Galilee, and asked him, “Sir, we wish to see Jesus.” Philip went and told Andrew; Andrew and Philip went and told Jesus. And Jesus answered them, “The hour has come for the Son of Man to be glorified. (John 12:20-23 ESV)
When the Greeks come to see Jesus, it is now the time for the Son of Man to be crucified. Then John makes clear the crucifixion is an event which will reach out to all people:
And I, when I am lifted up from the earth, will draw all people to myself.” (John 12:32 ESV)
The Fourth Gospel makes it clear salvation was for the Jewish people, Samaritans, and Gentiles.
I would point to Luke's record in Acts, where Paul's ministry to Gentiles is proven entirely legitimate.
Acts 9:15 But the Lord said unto him [Ananias, by whose hands Paul was initially filled with the Spirit], Go thy way: for he [Paul] is a chosen vessel unto me, to bear my name before the Gentiles, and kings, and the children of Israel:
There are lots more references that confirm Paul's acceptance and approval by the Apostles, and certify his unique calling to invite non-Jews into the true Commonwealth of Israel, for example:
Acts 11:18 When they heard these things, they held their peace, and glorified God, saying, Then hath God also to the Gentiles granted repentance unto life.
Acts 14:27 And when they were come, and had gathered the church together, they rehearsed all that God had done with them, and how he [God] had opened the door of faith unto the Gentiles.
Acts 15:14 Simeon hath declared how God at the first did visit the Gentiles, to take out of them a people for his name.
Acts 28:28 Be it known therefore unto you, that the salvation of God is sent unto the Gentiles, and that they will hear it.
In these and other similar texts, Gentiles (Hebrew "goyim") generally refers to all non-Jewish nations, leaving no doubt in my mind that salvation truly is for all.
Matthew records the Great Commission:
28:19-20 Therefore go and make disciples of all nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to obey everything I have commanded you. And remember, I am with you always, to the end of the age. [NET]
"Nations" is a reference to gentile nations.
The other evangelists record other instances of Jesus teaching that salvation is available to all.
Mark 16:15-16 He said to them, “Go into all the world and preach the gospel to every creature. The one who believes and is baptized will be saved, but the one who does not believe will be condemned. [NET]
Luke 24:46-49 and said to them, “Thus it stands written that the Christ would suffer and would rise from the dead on the third day, and repentance for the forgiveness of sins would be proclaimed in his name to all nations, beginning from Jerusalem. You are witnesses of these things. And look, I am sending you what my Father promised. But stay in the city until you have been clothed with power from on high. [NET]
John 20:21 So Jesus said to them again, “Peace be with you. Just as the Father has sent me, I also send you.” [NET]
To get the full impact of that verse, you have to look back at John 17:18
Just as you sent me into the world, so I sent them into the world. [NET]
Jesus' ministry started with the lost sheep of Israel, but it was always planned to go further out. We see this in the Canaanite woman, who recognizes that Jesus first came for the children but has grace to spare for those outside the Jewish faith and line (Matthew 15:21-28).
Luke makes sure his readers understand when he records Jesus at the Ascension further explaining the commission.
Acts 1:8 But you will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the farthest parts of the earth. [NET]
That statement stands as an outline of the book of Acts. First, they witness in Jerusalem, then Judea, then Samaria, and the book ends with Paul under arrest, preparing for trial (ch 28), and the disciples having "turned the world upside down" (Acts 17:6).
It may be taken in addition that Luke the evangelist was a gentile, a native of Syria. If salvation was not available to all, then 53 chapters of the New Testament would not exist. Perhaps it is because of his gentile background that Luke shows so much ministry to gentiles. For instance, in Acts 10, Peter receives a vision from God to preach to the gentiles, goes to preach to Romans, and sees them filled with the Holy Spirit at their conversion. As the circumcised believers are astonished that gentiles have received the gift of the Holy Spirit which can only come to the saved, Peter says,
No one can withhold the water for these people to be baptized, who have received the Holy Spirit just as we did, can he? (10:47)
A final example can be found in the book of Revelation 7:9-10, where a multitude of the saved praise God and the Lamb for salvation. These are from every nation, tribe, people, and tongue.
After these things I looked, and here was an enormous crowd that no one could count, made up of persons from every nation, tribe, people, and language, standing before the throne and before the Lamb dressed in long white robes, and with palm branches in their hands. They were shouting out in a loud voice,
“Salvation belongs to our God, to the one seated on the throne, and to the Lamb!” [NET]