In the letter to the Galatians, Paul makes the case that salvation comes to Gentiles through faith in Christ alone (Ehrman 348). From the letter we are able to deduce that other missionaries, claiming that observance of Torah as summed up in the rite of circumcision (Osiek 424), have arrived in Galatia, introducing new criteria for salvation. Based on what Paul says in the letter, it appears that these missionaries may also charge that Paul has tampered with the gospel received from Jesus’ own followers.  In response to these claims, Paul includes within the epistle a large passage of autobiographical information (Galatians 1:11-2:21), which can be broken down into several episodes, each of which is designed to lend support to Paul’s gospel of salvation through faith alone.
In Galatians 1:11-12 Paul first asserts that his gospel message comes not from humans, but rather from divine revelation received directly from Jesus Christ. A related claim is found in the letter’s opening where Paul claims that his apostolic mission is backed by both Jesus Christ and God the Father (Galatians 1:1). Assuming that Paul has credibility among the Galatian churches that he has founded, this claim not only explains the difference between his gospel and that of the Judaizers, but also asserts that he is backed by the ultimate authority.
Next Paul describes his earlier life in Judaism and his persecution of the nascent Church. As a zealous practitioner of his religion, advanced beyond many of his peers (Galatians 1:14), Paul defends his credentials as an authority in Judaism of no lesser standing than his opponents.  It is not for ignorance of Jewish law and customs that he has proclaimed a message devoid of Torah observance. Rather, again, he points to divine revelation.
Next Paul is clear that he does not return immediately to Jerusalem to discuss the revelation he received, but rather he travels to Arabia and returns to Damascus, biding his time for three years before making the trip to see Peter and James, the brother of Jesus (Galatians 1:17-19). Further, he does not share his message with the Judean congregations, but rather returns after a short visit to Syria and Cilicia (Galatians 1:22-23). Implicit in this account is Paul’s claim that he has not interacted with Jewish churches to receive his gospel, nor was he swayed from his own revelation by his interactions with Peter and James, both of whom were commissioned before him.
In Galatians 2:1-10 Paul claims that 14 years pass before he returns to Jerusalem again, this time with his friend Titus, a Gentile convert. During their stay in Jerusalem "false brothers" attempt to coerce Titus into circumcision, but Paul’s company stands strong in their convictions. Paul reports that Peter, James and John ultimately acknowledge his mission to the Gentiles as parallel to Peter’s own mission to the Jews. The apostles do not add to the gospel that Paul preaches, but only request that Paul take up an offering for the poor in Jerusalem while on his missionary journeys. Again Paul makes the claim that his gospel to the Gentiles remains uncontested by these apostles, the "acknowledged pillars" or the Jerusalem community.
Finally, in Galatians 2:11-14 Paul recounts his conflict with Peter in Antioch. In his story, he tells that Peter also relaxes his observance of Torah in order to eat with Gentiles (presumably Paul’s friends and converts to his gospel). But when Torah observant men arrive from James, Paul claims that Peter forsakes communion with the Gentiles out of fear, thus undermining Paul’s gospel claim that the Gentiles are on equal footing by faith with the Jewish Christ followers. Paul publicly rebukes Peter, asking how Peter can expect Gentile converts to observe the Torah when even he himself not. Through this account Paul hopes to show that even Peter, when unintimidated by fellow Jews, does not believe that observance of Torah is required by the gospel of Jesus Christ. 
Thus, through his claims of direct revelation from Jesus Christ and divine commissioning Paul denies that he has preached a perverted gospel. And by relating his encounters with several Jerusalem apostles, he at once denies that he requires their endorsements, while at the same time pointing out that they do not object to his missionary work. Establishing his bona fides, Paul is free to argue again for his gospel of salvation to the Gentiles by faith alone.
Ehrman, Bart D. The New Testament: a historical introduction to the early Christian writings. 4th Edition. New York: Oxford University Press, 2008.
Osiek, Carolyn. "Galatians." Women’s Bible Commentary. Ed. Carol A. Newsom and Sharon H. Ringe. Expanded Edition. Louisville: Westminster John Knox, 1998. 423-427.