No, it is not the gospel, rather, it is a summary reminder of the gospel.
As is obvious throughout Paul's writings and throughout Luke's depiction in Acts of Paul's words and actions, the gospel of Christ involves educating a person about what sin is; about what its eternal consequence is; about Who Christ was; about how He was God's solution to the eternal consequence of sin; about how that the person needs to avail Himself of this, God's solution in His Son, by Believing this gospel or good news of Christ.
Paul was writing to people whom Paul himself every now and then asserts he is reminding they had not even known of God, nor of their sinful condition before said God, nor of Christ, and God's solution in Him.
What you have done, and what others who claim that 1 Corinthians 15 is all the gospel is, is you have read what you have read in Scripture, and or have been taught about what I have described (as described in Scripture) INTO Paul's words in 1 Cor. 15.
But 1 Cor. 15 is merely a reminder of much more that Paul had taught them about the gospel - it is not all that the gospel is.
It is more like a short exchange between two people who have been much more informed on the details of a thing, and or have agreed on the content of a much longer, much more detailed description of a thing.
Later, in conversation, when one of them says "Remember the Alamo," for example, or "Remember what happened at the Alamo" the phrase sets off in them a memory of all that phrase is actually referring to.
Sort of like the phrase "911" it not only no longer means alone, what it used to before "911," but now sets off in one a memory of all the horrific events of that day, in one phrase - "911."
Etc.
1 Cor. 15 is like that.
Paul is addressing people whom he had had to educate on all those details.
Romans 15:20 Yea, so have I strived to preach the gospel, not where Christ was named, lest I should build upon another man's foundation: 15:21 But as it is written, To whom he was not spoken of, they shall see: and they that have not heard shall understand.
I suppose that when Paul reminded the Corinthians of the following, that that was all there had been to say about it to them, when he first met them? No, it is not.
1 Corinthians 12:2 Ye know that ye were Gentiles, carried away unto these dumb idols, even as ye were led.
As in the latter half of Acts 17, Paul would educate these once God-LESS Gentiles on MANY details concerning "the gospel."
In fact, whenever HE uses the phrase "the gospel" this one phrase by itself VERY OFTEN throws people off who have not "been taught" ALL that PAUL means by it.
Automatically THEIR response is - "oh, what Jesus preached, okay..."
Years in that, and forget about it - trying to get through to them about this error of theirs like is pulling teeth with a pair of pliers made out of taffy, lol.
Those who assert that 1 Cor.15 alone is the gospel of Christ are simply wrong.
Acts 17: 11, 12.
Very good and very well. It is a summary and a reminder of the gospel, but not all that can be known.