Irenaeus wasn't "wrong" to call them Gnostics, just like he wasn't "wrong" to call them heretics.And the key point is that Irenaeus was wrong to do that
If want to get even deeper into it, Adam and Eve would be the first gnostics because they ate from the tree of Knowledge of Good and Evil:
And Jehovah God took the man, and put him into the garden of Eden to dress it and to keep it. And Jehovah God commanded the man, saying, Of every tree of the garden thou mayest freely eat: but of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.
And what happens in the very next chapter? Eve is persuaded by the serpent to eat from the tree, who then persuades Adam to eat from it, thus gaining knowledge and knowing they were naked:
And the eyes of them both were opened, and they knew that they were naked; and they sewed fig-leaves together, and made themselves aprons.
And it is precisely this scene in particular that seems to have inspired a whole sabaoth of "gnostic" sects, like the Sethians and the Cainites and Adamites and the Valentinians.
So it isn't like Irenaeus was just reaching for a term and making it up. To say Irenaeus was "wrong" is to presume some anti-historic bias onto these men writing in their own time.
Napoleon wasn't "short" for his day, but it isn't "wrong" to call him short today. Historically inaccurate. But not "wrong".
I lost my train of thought.