Short answer
Not in the usual Christian sense. In most Gnostic texts, the fundamental human problem is ignorance of one’s divine origin and entrapment in the material world, rather than sin understood as willful disobedience that incurs guilt. Salvation is therefore described as knowledge (gnosis) that awakens a person to what they truly are. That said, Gnostic groups varied a great deal, and many of them still had ethical teaching.
Ignorance as the core problem
In the Gnostic texts, the world is typically presented as a place where the divine spark is forgotten. The Gospel of Truth, a Valentinian text found at Nag Hammadi, describes ignorance of the Father as the source of anguish and error, and it describes the coming of knowledge as dispelling it. The emphasis is therefore on awakening rather than on forgiveness of wrongdoing. For a broader introduction to the teachings, see Gnosticism and esotericism.
The fall of Sophia and the flawed creation
Many Gnostic myths describe a cosmic mistake, often told through the figure of Sophia (“Wisdom”). In several Sethian texts, such as the Apocryphon of John, Sophia’s error leads to the birth of the flawed creator, the demiurge, who makes the material world. The “fall” here is a cosmic event that precedes humanity, and the human predicament is the result. This differs from the Genesis-based Christian account in which human disobedience brings sin and death into the world. Some Gnostic texts reinterpret the Genesis story, even presenting the serpent in a positive light, though not all do.
Did Gnostics care about behavior?
Yes, in many cases, though the form varied. Some groups appear to have stressed asceticism, meaning abstinence from the body’s pleasures, because the body was seen as part of a flawed world. Valentinian writers described spiritual people as those who ought to live differently because of their knowledge. The surviving texts include ethical exhortations, though less systematically than in orthodox Christian writing.
Accusations of libertinism
Early Christian opponents, such as Irenaeus and Epiphanius, accused some Gnostic groups of immoral behavior, claiming that they thought the spiritual elect could not be harmed by anything they did. Scholars treat these reports cautiously. They come from hostile writers, and the Nag Hammadi texts do not generally support them. Some groups may have held antinomian views, but the picture is unclear, and it would be wrong to assume that all Gnostics did.
Contrast with orthodox Christian sin
In mainstream Christianity, sin is generally understood as disobedience to God, with guilt, and forgiveness comes through Christ’s redemptive work. In Gnostic texts, the savior typically brings knowledge and awakens, rather than atoning for guilt. The material world is also often seen as the problem, whereas orthodox Christianity affirms creation as good. This is a broad contrast, not a perfect rule, since Gnostic and Christian terms overlap in many texts.
Caveats about the sources
Our knowledge of Gnosticism comes from two kinds of material: the polemics of opponents and the original texts found at Nag Hammadi in 1945, which are often fragmentary and diverse. Scholars still debate whether “Gnosticism” is a single category at all. So any one-line answer to “is there sin in Gnosticism?” will simplify. If you want to read the wider context, Western esotericism places Gnosticism among related currents.