Galatians

Galatians 1:11-12
For I want you to know, brothers and sisters, that the gospel that was proclaimed by me is not of human origin; for I did not receive it from a human source, nor was I taught it, but I received it through a revelation of Jesus Christ. (NRSV)

Quoting Longenecker in his commentary on Galatians:

…as for Gentiles being accepted by God and living as Christians apart from the regulations of the Mosaic law–and so, as for the legitimacy of a Gentile mission apart from the Jewish law–Paul saw this as a ‘mystery’ enigmatically rooted in the prophetic Scriptures but now made known to him by revelation (Rom 16:25; Eph 3:2-10; Col 1:26-27), and so uniquely his.

…what they [the Judaizers]…opposed were the implications Paul drew from these confessions for a law-free gospel among Gentiles. Paul, however, saw in the proclamation of full salvation in Christ the attendant truth of acceptance and life for Gentiles apart from the Mosaic law. This is what he calls ‘the gospel I preached to you’ or ‘my gospel’ (Rom 2:16; Rom 16:25; see also 2 Tim 2:8; and ‘our gospel’ at 2 Cor 4:3; 1 Thess 1:5; 2 Thess 2:14).

The Judaizers (those seeking to have those in Galatia circumcised and completing their salvation by obeying the law) claimed that Paul didn’t have the authority to preach his gospel and also that he initially received his teaching from the “real” apostles in Jerusalem as opposed to direct revelation from Jesus Christ.

Galatians 1:18
Then after three years I did go up to Jerusalem to visit Cephas and stayed with him fifteen days;

Although Paul did see Peter (Cephas) for 15 days, as Longenecker says with a bit of humor:

Certainly their fifteen days together were not spent ‘talking about the weather.’ They discussed, without a doubt, matters pertaining to their common commitment to Christ. And it is not beyond the range of reasonable probability to believe that such discussions included Peter’s accounts of Jesus’ ministry, and that from such accounts Paul learned much. But to learn about the details of Jesus’ earthly life from Peter and to be subordinate to or dependent on Peter for his apostleship and Gentile mission are clearly quite different matters. Paul is willing to acknowledge the former, but he is adamant in his rejection of the latter.

Paul is not bragging or trying to be a renegade apostle going off on his own. He is only defending the authority to preach the gospel given him and its authenticity.

This is well illustrated in Ephesians (also cited by Longenecker above). O’Brien points out that the words given/gift and grace are mentioned three times each in this passage for emphasis:

Ephesians 3:2-8
for surely you have already heard of the commission of God’s grace that was given me for you, and how the mystery was made known to me by revelation, as I wrote above in a few words, a reading of which will enable you to perceive my understanding of the mystery of Christ. In former generations this mystery was not made known to humankind, as it has now been revealed to his holy apostles and prophets by the Spirit: that is, the Gentiles have become fellow heirs, members of the same body, and sharers in the promise in Christ Jesus through the gospel. Of this gospel I have become a servant according to the gift of God’s grace that was given me by the working of his power. Although I am the very least of all the saints, this grace was given to me to bring to the Gentiles the news of the boundless riches of Christ,