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:
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).