[Natural Law]

"In the book of Acts, chapter 9, Paul was given a specific message of what he was to preach so there was not a forgetting of the Gospel or any question of invalidating them."

Exactly! The letters of Paul were not written a priori. This is where both Luther and Calvin went wrong. They are meant ONLY to support, clarify, and reinforce the contents of the Synoptic Gospels. That alone was Paul's mission. They are not stand alone additions, revisions, extensions or new revelations and not to be used out of the context of the Synoptic Gospels.

[count-your-change]

And the reverse is just as true. The gospel accounts were part of scripture and so were Paul's letters but all had one ultimate author.
In Acts 13:26-40 Paul preached the gospels and as Acts 9:19-22 he proved to Jews that “logically, this was the Christ”.

Whether by recounting Jesus’ life or application of the prophetic scriptures, it was the unfolding of what the good news was that Peter said even the angels were desirous of peering into. (1 Peter 1:10-12)

Not Luke's good news or Paul's, etc., but one gospel.