For me, some differences in gnostic thought and Paul's thought:
1. Who was the God of the Jews? For gnostics: the Demiurge. For Paul: the True God.
2. Did Adam transgress against the True God? For gnostics: No. For Paul: yes.
3. What was the outcome of Adam's rebellion? For gnostics: the serpent awakened knowledge in Adam and this was a good thing. For Paul: Adam transgressed and brought death into the world. This was a bad thing.
Now, it may be that the passages I'm using for Paul are not original to Paul. So this is really the start of a broader conversation, I guess.
I don't know that any of this can be assumed.
I guess firstly we should just stop saying "Paul" anything. I for one do think there was a real Paul, but at this point the Pauline letters are a jumble of conflicting ideas, clearly written by multiple people. Anyone claiming to be able to distill the "real Paul" out of them is either fooling themselves or trying to fool others.
Personally, I use two points of reference to try and identify at least an earlier layer. Passages that have parallels in the Gospel of Mark or that are testified to have existed in Marcon's version of the letters I take as at least prior to proto-orthodox revision. But even that is not necessarily "original". After all, Marcion's collection included Colossians and "Ephesians", which are clearly not written by Paul, so it is clear that Marcon's collection was not a fully authentic collection of the writings of a single person. And Colossians and "Ephesians" also espouse ideas that conflict with the other letters, so it wasn't just a different person, but a person with a different theology. So if forged letters with a different theology existed in Marcion's collection, then surely there could also have been revisions to the other letters as well.
So it seems to me that Marcion's layer precedes the proto-orthodox layer, but Marcion's layer is itself already "corrupted" by multiple authors.
So anyway, trying to claim that "Paul" said this or that, or thought this or that just doesn't work. But we can identify strains of thought and connected theologies within the letters. We don't know how many different strains of thought exist in the letters and its not clear what the boundaries are around each of the different theologies. Clearly the letters contain a mix of sometimes conflicting theologies.