Paul referred to the gospel (Greek euangelion, good news) several times in his epistles in the first person possessive; that is, he referred to it as "my gospel" (Rom. 2:16; Rom. 16:25; 2 Tim. 2:8). Probably this was partly to distinguish it from false gospels (see Gal. 1:6-9; 2 Cor. 11:4), but it also demonstrates that he had a sense of ownership of the message he proclaimed. True, it was ultimately the gospel of Jesus Christ (Mark 1:1), because it was about Jesus Christ, and the gospel of God (Rom. 1:1), because it came from God. Nevertheless, as Paul was the earthly messenger by which that gospel was delivered, there was a valid sense in which it could be described as Paul's gospel - that is, the good news which had become known across the Roman world through the preaching of Paul.
For me, one of the striking characteristics of Paul's gospel is the paradox that it was at once both very simple and very complex. As to the gospel's simplicity, Paul proclaimed to the Corinthians:
"1 And I, when I came to you, brothers, did not come proclaiming to you the testimony of God with lofty speech or wisdom. 2 For I decided to know nothing among you except Jesus Christ and him crucified. 3 And I was with you in weakness and in fear and much trembling, 4 and my speech and my message were not in plausible words of wisdom, but in demonstration of the Spirit and of power, 5 so that your faith might not rest in the wisdom of men but in the power of God." (1 Cor. 2:1-5)
Notice that Paul decided to know nothing except Jesus Christ and him crucified. It was an active choice. Paul had been educated at the feet of a leading Jewish scholar, Gamaliel; he was a learned man and had seemingly been something of a theological prodigy in his youth (Acts 22:3; Gal. 1:14). He understood the Hebrew Scriptures very well and was well placed to marvel, "Oh, the depth of the riches and wisdom and knowledge of God! How unsearchable are his judgments and how inscrutable his ways!" (Rom. 11:33).
Paul's epistles are masterpieces with great theological complexity, even from a purely human standpoint. However, for the sake of the Corinthians he emphasized the gospel in its simplicity, possibly because he wanted them to focus less on knowledge and more on love (1 Cor. 8:1-3; 1 Cor. 13:2).
In the final analysis, what mattered to Paul was the gospel through which his flock could be saved: "that Christ died for our sins in accordance with the Scriptures, that he was buried, that he was raised on the third day in accordance with the Scriptures" (1 Cor. 15:1-4; cf. Rom. 10:9).
If you are preaching the gospel, it is good for you to know and understand the Scriptures in as much depth as possible. Further, there is a need to contextualize the message for your audience, as Paul did in Athens (Acts 17:16-32). However, when preaching your gospel to those with little biblical background knowledge (i.e. most people), resist the urge to appear wise by presenting the gospel in all its theological depth. Don't judge the unchurched for having little interest in the history of the kingdoms of Israel and Judah, the precise mechanism of the atonement, or the details of the millennial age. To the weak (in knowledge, or desire for such knowledge) become as weak, in order to win them (1 Cor. 9:22).
In short, when preaching your gospel, refrain from any level of complexity that will take your hearers' focus away from "Jesus Christ, and him crucified!"
