If you define it literally, you can easily find "Gnosticism" (personal knowledge/revelation) in the Bible itself (e.g. Mt 11, Mt 16, Lk 2, Jn 16, 2 Tim 3, all of Rev).

But we generally agree to only label it as Gnosticism if it doesn't pass the consistency trial (2 Pet 1, 1 Jn 4), and especially if it outright fails it.