The Judaizers were evidently claiming that Paul had not received his gospel directly from Christ, and that consequently parts of what he taught were wrong. They tried to prove that there was some human agency involved in his commission which left room for the alteration of the message.
Paul’s defence of his preaching here is not a defence which an ordinary church member or even a pastor could use today. We point to an inspired Bible and to the apostolic authority, but there are no apostles today; none who have seen the risen Christ, been personally appointed by him, receive revelation from him, and been enabled to perform sign miracles. Many claim today that God speaks his truth directly through them, but can they match the qualifications that Paul advances in support of his authority? For us it is enough that our gospel is built on the foundation of the apostles and prophets as recorded in Scripture. It does not matter that the gospel has reached us through the agency of men, but it certainly matters that those who were the first to teach it received it directly from God.
The vast majority of the elect receive the gospel via another human being – a preacher, a portion of Scripture (written by human apostles or prophets), a tract, or a witnessing friend. Why was it so important for Paul to prove that it was different in his case? If he was to speak with that authority and independency that is required of an apostle, then his gospel must also have come to him by revelation. If this message is really of divine origin then it was necessary at some point that it should have been revealed directly by God.