After reading some verses regarding Paul, my conclusion is Paul already preach to the gentiles before Barnabas went to Tarsus to meet him (then brings him to Antioch).
But when God, who set me apart from my mother's womb and called me by his grace, was pleased to reveal his Son in me so that I might preach him among the Gentiles, my immediate response was not to consult any human being. I did not go up to Jerusalem to see those who were apostles before I was, but I went into Arabia. Later I returned to Damascus.
The verse above is one of the reason of my conclusion.
But my "advanced" Christian friend told me that the first time Paul preach to the gentiles was after Paul meet Barnabas in Tarsus. My friend gave me Acts 13:46
Then Paul and Barnabas answered them boldly: "We had to speak the word of God to you first. Since you reject it and do not consider yourselves worthy of eternal life, we now turn to the Gentiles.
Which means, Paul never preached to the gentiles before he met Barnabas in Tarsus.
Now I wonder why did Paul "delay" his preaching to the gentiles :).
Addition after I read "How should the chronologies of Galatians 1:16-17 and Acts 9 be reconciled?" ----> neither the question nor the answer tell whether Paul preach to the Gentiles or not when he is still "alone" (without Barnabas).
For some days he was with the disciples at Damascus. And immediately he proclaimed Jesus in the synagogues
It seems to me that during in Damascus - Paul did preach to the Jews in the Synagogues. But my question remains : "did Paul also preach to the gentiles besides preaching in the synagogues ?"
After 15 days Paul in Peter's house he said that he went to Syria and Cilicia (Gal 1:16).
In Acts 9:30
When the believers learned of this, they took him down to Caesarea and sent him off to Tarsus.
I assume that both verses tell the same event which is after 15 days Paul in Peter's house in Jerusalem. But since both verses didn't mention Barnabas, I assume that Paul leave Jerusalem (after the 15 days in Peter's house) without Barnabas.
Then my question arise : (after departed from Jerusalem, Peter's house) what did Paul do after he arrived at his destination ? Did he preach or didn't preach at all there ? If he preach there (before Barnabas see him in Tarsus then bring him to Antioch), did he only preach to the Jews ? or did he also preach to the Gentiles ?