The origins of Gnosticism are disputed by modern scholars, but even if Gnosticism had a wholly non-Jewish origin, that would hardly make it irrelevant to early Christianity since Christianity syncretized Judaism and Greco-Roman religious themes. After the Jewish Temple was destroyed and Christianity was absorbed into the Roman Empire, the pagan aspects of Christianity were emphasized. See for example, the polytheistic doctrine of the Trinity.
Anyway, as the Wikipedia page on Gnosticism points out, “The proto-orthodox Christian groups called Gnostics a heresy of Christianity, but according to the modern scholars the theology's origin is closely related to Jewish sectarian milieus and early Christian sects…
“Many heads of gnostic schools were identified as Jewish Christians by Church Fathers, and Hebrew words and names of God were applied in some gnostic systems. The cosmogonic speculations among Christian Gnostics had partial origins in Maaseh Bereshit and Maaseh Merkabah. This thesis is most notably put forward by Gershom Scholem (1897–1982) and Gilles Quispel (1916–2006). Scholem detected Jewish gnosis in the imagery of the merkavah, which can also be found in "Christian" Gnostic documents, for example the being "caught away" to the third heaven mentioned by Paul the Apostle. Quispel sees Gnosticism as an independent Jewish development, tracing its origins to Alexandrian Jews, to which group Valentinus was also connected.”
You can find numerous Gnostic ideas in Paul’s epistles and in the Gospel of John, so Gnosticism is hardly irrelevant to the essence of Christianity.