What was Paul's "revelation" (mentioned in Galatians 2:2)? Then after an interval of fourteen years I went up again to Jerusalem with Barnabas, taking Titus along also. It was because of a revelation that I went up; and I submitted to them the gospel which I preach among the Gentiles.  - Galatians 2:1-2, NASB


I am wondering what Paul's revelation was?  Do we have any scripture, tradition, or writings from church fathers which might help answer this?
The revelation was most likely simply a directive from God to go up to Jerusalem to talk with the apostles. This is the most natural way to read the verse, and there would be nothing surprising about this conclusion on the basis of the rest of Paul's life, since he was an apostle.

An apostle had to be someone who had seen Christ directly with his eyes. This is evidenced in the choosing of a replacement for Judas. It is also shown in this chapter itself; Paul is arguing that he has apostolic authority on the basis of the fact that he has had a revelation of Jesus Christ (1:11-12). Thus, he is an apostle not because he was sent by men, nor even commissioned by (authorized) men on behalf of God (this is the usual interpretation of 1:1). See also 1:16. Thus Paul says of himself elsewhere, "Last of all, as though I had been born at the wrong time, I also saw him."

Other examples of specific revelations he received:


His conversion story, when he sees Christ face-to-face
His apparent reference to himself in the "I know a man who has caught up into third heaven" passage
His distinction in 1 Corinthians 7 between his own apostolic wisdom and the revelation he has directly from Jesus
The Spirit's revelation to him of his sufferings in Jerusalem
The Spirit's preventing him from going into Asia at one point (there may be a different interpretation of this)


Therefore, given that the Lord seems to have spoken to him not only at his conversion and not only to teach him doctrine, and given that the context is that he had been gone for fourteen years without seeing any need to go to Jerusalem (2:1), it seems extremely likely that the revelation was a specific direction that he ought to go up to Jerusalem and confer with the apostles, particularly James, Peter and John (2:9).