We don't really know if Gnostic is a label that "tars everybody with the same brush" or not. We do know that it was only used to label certain sects of heretics. It's hard to say whether this was polemical or for a good reason, because we only really have one side of that story.
Early literature that was written contemporary with the Gnostics was probably more like argumentation that happened within the same church spaces. They were arguing over the proper way to regard scripture. The persecution didn't happen until a bit later. "Heresy" at this point just referred to a school of thought, and didn't have the connotations that it does today.
Irenaeus first established the doctrine of four gospels and no more, with the synoptic gospels interpreted in the light of John. Irenaeus' opponents, however, claimed to have received secret teachings from Jesus via other apostles which were not publicly known. Gnosticism is predicated on the existence of such hidden knowledge, but brief references to private teachings of Jesus have also survived in the canonic Scripture as did warning by the Christ that there would be false prophets or false teachers. Irenaeus' opponents also claimed that the wellsprings of divine inspiration were not dried up, which is the doctrine of continuing revelation.
- wiki Heresy_in_Christianity -
This was the beginning of so-called orthodoxy. The Gospel of John was being emphasised.
The Gospel of Thomas, it is often claimed, has some Gnostic elements but lacks the full Gnostic cosmology. However, even the description of these elements as "gnostic" is based mainly upon the presupposition that the text as a whole is a "gnostic" gospel, and this idea itself is based upon little other than the fact that it was found along with gnostic texts at Nag Hammadi. The scene in John in which "doubting Thomas" ascertains that the resurrected Jesus is physical refutes the Gnostic idea that Jesus returned to spirit form after death. The written gospel draws on an earlier oral tradition associated with Thomas. Some scholars argue that the Gospel of John was meant to oppose the beliefs of that community
- wiki Diversity_in_early_Christian_theology -
..so these "orthodox" Christians were bullies, who asserted their version, while actively oppressing
any other views. I find that highly suspicious, and while I am not accusing anybody of insincerity per se,
I think that people were being manipulated, and history of the early Christians was
eventually suppressed by the victors.[ Roman Nicean authorities ]