Even Irenaeus in his tirade against Gnostics never once comes out and says, “Hey, these Gnostic pests hate the flesh”. His problem with them was not that they despised or hated the flesh but that they denied it’s salvation. To support the salvation of the flesh is not to love the flesh anymore than to deny the salvation of the flesh is the hate it. The truth is, modern scholars are lazy sometimes, so they stick with this tag line: “Gnostics hated the flesh”, so they can sell really poorly written books their peers wouldn’t even dare read. This is because they have run out of good material most likely.
The early Gnostics were said to despise (not care for) the flesh. This means the last priority of theirs was to keep their body in perfect shape. This is very much opposed to Greco-Roman athletic culture in which the body was obsessed over. In fact, in Paul, we find both stances. It is possible that when referring to the body and conditioning it, he is referring to the church, or one’s not authentically Paul and the other is. I have read convincing arguments for both sides and even thought of my own.
The Stoics taught to despise the flesh as can be found in the Encheiridion of Epictetus transmitted by his pupil. There are several other platitudes that get thrown around on the internet and in books regarding Gnostic beliefs but the hatred for the flesh is by far and away the most common. It is lauded as fact but it’s merely tons of people copying one or two people who read early sources and had poor reading comprehension.
(Also note that it was not a common thing in antiquity to think someone wrote a work simply because their name was attached to it! It is common practice now to dismiss anything with a name affixed as not being by that author. In fact, this is done with virtually every single religious writing in Christianity and Gnosticism but hardly ever with any other religion or secular writers. There is no real set of pre-requisites or criteria for disproving authorship.)
It is not hard to see that Augustine was the flesh hater who used Plotinus and Pseudo-Dionysus.