What was mistaken about the 'gospel' that Paul opposed?
As I have often noted on here, regardless of who is asking what - the problem with such types of questions is the expectation no matter who attempts to answer it, that said individual will being doing so from where it's asker, is asking it, or looks at such things from.
In IP's case, one of his recurrent patterns (pattern observable in and through another's repeated words and or behavior, when objectively observed) is the often easily observable fact that he has long since made automatic habit, his taking of what he thinks a thing is saying is...as being what it is actually saying.
The man is simply too far gone in this hodg-podge dream-like patch-work result of his own readings into a thing, to really answer him to his satisfaction, outside of approaching these things in a manner as incompetent as his own.
Again - the problem with such types of questions is the expectation no matter who attempts to answer it, that said individual will being doing so from where it's asker, is asking it, or looks at such things from.
No matter who asks such questions.
The fact of the matter is that such questions jump the gun of what their asker first needs to both lay out and prove the soundness of, before asking their question.
That is the recurrent pattern in Scripture, other than when the particular authority asking the question is depicted asking their question of one who is already familiar with where said asker is asking their question from, to begin with.
In this, asking such questions of another in the manner IP has done, is the mark of an individual unfamiliar with the above practice depicted in Scripture.
Absent of the above, such are foolish, that is to say; unlearned questions.
And "books learned" too often results in one's having remained "unlearned."
Too often results in one who is - "Ever learning, and never able to come to the knowledge of the truth" - 2 Tim. 3:7.
In short...
"To the law and to the testimony: if they speak not according to this word, it is because there is no light in them" - Is. 8:20
Rom. 5:8
Acts 17:11,12