1 cor 15:1-3, for sure is Gospel Truth, but its a mistake to think thats all to it. The Gospel of My Lord Jesus Christ is much more informative of the overall Person and Work of the Jesus Christ. Its comprised of who He is, what He has accomplished, and for whom He did accomplish it for and the results and effects of His work. Also the Gospel that Paul was revealed had an element of Mystery. Paul often referred to it as a Mystery i.e Eph 6:19
And for me, that utterance may be given unto me, that I may open my mouth boldly, to make known the mystery of the gospel,
And that Mystery is not even discussed in 1 cor 15:1-3, Just simply stating that Christ died for our sins according to the scripture and that He was raised according to the scripture is by no means preaching the Mystery of the Gospel.
Paul frequently described the gospel and its implications as the Mystery of which one was the equality of both Jew and Gentile as Co Heirs and Co Partakers of the Gospel promises made to Israel.
Eph 3:
1For this cause I Paul, the prisoner of Jesus Christ for you Gentiles,
2If ye have heard of the dispensation of the grace of God which is given me to you-ward:
3How that by revelation he made known unto me the mystery; (as I wrote afore in few words,
This was all part of the Gospel that Paul had revealed to Him in 1 cor 15:
3For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
But he did not go into exhaustive detail in the 1 cor 15 passage about that aspect of the Gospel, as he did in romans 11.
4Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)
5Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;
6That the Gentiles should be fellow-heirs, and of the same body, and partakers of his promise in Christ by the gospel:
So its fine to say that 1 cor 15:1-3 is the gospel, but don't try to make it seem like nothing else has not been revealed as the Gospel too, it shows how unlearned one is of the Truth, and lack of study of the scriptures.
There is much more that is the Gospel in the scripture, but this will suffice for now to make the point.