The Cross and the Gospel

17 ¶ For Christ did not send me to baptize, but to preach the gospel, not in cleverness of speech, so that the cross of Christ would not be made void (1 Cor. 1).

The preaching of the cross is the preaching of the gospel.

23 but we preach Christ crucified, to Jews a stumbling block and to Gentiles foolishness,24 but to those who are the called, both Jews and Greeks, Christ the power of God and the wisdom of God (1Cor.1). While verse 17 says Paul was sent to preach the gospel, verse 23 says he preached Christ crucified, the power and wisdom of God.

1 ¶ And when I came to you, brethren, I did not come with superiority of speech or of wisdom, proclaiming to you the testimony of God. 2 For I determined to know nothing among you except Jesus Christ, and Him crucified. 3 I was with you in weakness and in fear and in much trembling, 4 and my message and my preaching were not in persuasive words of wisdom, but in demonstration of the Spirit and of power, 5 so that your faith would not rest on the wisdom of men, but on the power of God (1 Cor.2).

Paul's testified of Christ and him crucified. That was his message and preaching to the Corinthians.

Paul became quite explicit in explaining to the Corinthians what his message was:

1 ¶ Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand,
2 by which also you are saved, if you hold fast the word which I preached to you, unless you believed in vain.
3 For I delivered to you as of first importance what I also received, that Christ died for our sins according to the Scriptures,
4 and that He was buried, and that He was raised on the third day according to the Scriptures,
5 and that He appeared to Cephas, then to the twelve (1Cor. 15).