Who was John?
Well, he was a crazy hermit who lived in the desert dousing people with water and saying that God talked to him. He became a distraction to the authorities, and they lopped off his head.
What is his significance to Gnostics?
Well, we may not live in the desert, but we continue dousing people with water and saying God talks to us. And the authorities are all distracted by the gnostic movement (just look at all the da vinci code backlash), and may try to lop of our heads at some point.
So, there are parallels.
Matthew 3 lays out his message:
"7You brood of vipers! Who warned you to flee from the coming wrath? 8Produce fruit in keeping with repentance. 9And do not think you can say to yourselves, 'We have Abraham as our father.' I tell you that out of these stones God can raise up children for Abraham. 10The ax is already at the root of the trees, and every tree that does not produce good fruit will be cut down and thrown into the fire.
11"I baptize you with water for repentance. But after me will come one who is more powerful than I, whose sandals I am not fit to carry. He will baptize you with the Holy Spirit and with fire. 12His winnowing fork is in his hand, and he will clear his threshing floor, gathering his wheat into the barn and burning up the chaff with unquenchable fire."
So, in an esoteric setting, St John calls us to turn away from worshiping the material, which cannot save us, and towards the gnosis, which can (fruit is referred to as spiritual fruit elsewhere). The stones are the unenlightened, the children of abraham are the enlightened. Water teaches us to turn from our past non-virtues, and wash away what holds us back. Fire gives us the energy to move forward, and attain the light which we seek. The non-virtue is the chaff, the wheat our own spiritual growth.
Most importantly, to me, John shows us that THIS IS NOT AN EASY PROCESS. There are axes, and floods, and fire, and wrath. I see this echoed all throughout hermetic literature: Crowley mocking his students, Bardon's axiom: "Be kind and generous to your fellow man, but merciless and demanding toward yourself." It takes work, and like I posted on the Palm Tree Garden: If you're not willing to see this to the end, you're better off to stop now, before you get time and energy invested in it.
A parable from my own life: In New Orleans, I met a prophet, and his student. The student bet the prophet $50 that the student could give up beer for a month. The prophet was confident the student would fail. Well, 25 days into the bet, the student pays the $50 and goes and has a beer. The prophet said "Why did you go through all this? You knew you wouldn't last to the end, I even told you so. Why didn't you pay the $50 on the first day, and then you'd be having beer for the past 24 days. Better yet, why make the bet at all?"
This is not to say there won't be up and down cycles, but even in those cycles, you need committment.
John shows us the life giving water. Jesus shows us the life affirming fire. The Christ is that water and fire, manifest within us. The examples of Jesus, John the Baptist, and the apostles show us committment. Look at the lives of the saints: How many died of natural causes? Interestingly, only John the Evangelist. That's commitment.
Luckily, we're not literally executed these days for what we believe in, at least not usually. Metaphorically, but in a very real way as well, we have to die to the way we were, and commit to a resurrection as new people, filled with the spirit.