Disclaimer: Attempting to define Gnosticism is probably impossible. People adhering to different established religions, or to none at all, can be Gnostics. There are a few current Gnostic churches, but their specific approaches can’t be said to represent true Gnosticism any more than previous incarnations of Gnostic thought. I am not an expert.
With that out of the way... At its most basic, Gnosticism is a religious perspective. Although Gnosticism is hard to define, there are a few generally accepted characteristics of Gnosticism. First, the world that we experience with our senses is an imperfect creation. Second, the creator of this faulty world is also imperfect. Third, God is not this creator. Fourth, everything, including the creator of our world, is formed from the Light of God and contains a spark of that Light. Fifth, humans are special in that this spark of Light can be brought into our consciousness. Sixth, as our consciousness becomes more enlighted (receives and processes more gnosis) we become closer and closer to our original unification with God. This is the mission of our lives. Seventh, none of these characteristics are to be taken literally. They are meant only as an expression of our situation here on earth.
Gnosis, in Greek, means “to know”. It doesn’t mean to know something through our senses (“this milk is sour”) or through reason (2+2=4). It’s a kind of knowing like when you love somebody. In what way can we describe that knowledge? We can say that love feels like this or feels like that, but the feeling isn’t the same as the knowing (someone considering marriage might say, “I feel like I love him, but I don’t know if I love him”). Like love, we can’t explain gnosis because it’s in the “heart”, not the mind. Its best expression is through art, music, or myth – communications that can transfer gnosis directly from one “heart” to another without the mind’s interference.
What do Gnostics know? They “know” that spark of God. It comes either to them or out of them and is raised into their consciousness. It comes in innumerably variable ways. This experience is the common thread through all Gnosticism. It can’t be proven. It can’t be observed. It can’t even be properly described. Frankly, the mind is likely to initially reject any discussion of gnosis as hogwash. But Gnostics say that “mind” comes from the creator, while Light comes from God.
If these ideas seem somehow familiar to you even though you’ve never heard of gnosis or known what it meant, maybe there was a little spark. It’s something to consider and it’s something only you can know.