The way Paul describes his gospel is a bit unusual, but it serves his purposes well. He describes his gospel as τὸ εὐαγγέλιον τὸ εὐαγγελισθὲν ὑπʼ ἐμοῦ, "the gospel which was preached by me." There are two important observations to make here. First, Paul describes his gospel with an aorist tense adjectival participle, but his central assertion about it ("it is not according to man") uses the present tense when one might have expected him to use the imperfect, referencing the past time of his preaching. With this switch from aorist to present, Paul juxtaposes what he proclaimed in the past with the present state of that proclamation. Essentially Paul maintains that at the time he preached to the Galatians up to the present his gospel cannot be described as being according to man. In other words, then and now his gospel has retained its divine character. Second, the fact that Paul uses the passive participle plus "by me" is unusual. Compare this to 1 Cor 15:1: "I want you to know, brothers, the gospel which I preached to you," which is a much more natural phrasing. The difference in voice can be attributed to the differing occasions between the two statements. In 1 Cor 15 Paul is discussing the common confession he and the Corinthian church share. There is no dispute over the gospel which he proclaimed and they accepted; their issues lie elsewhere. Not so in Galatia. Because the point of contention was precisely over the gospel Paul preached, he needed to focus on its divine origin. He use of the passive participle plus "by me" enables him to move the focus off himself ever so slightly. Yes, he was the one who proclaimed it, but he was not its ultimate source or origin.