Some Gnostics believe that we're here to be farmed for our sorrow, feeding an entity this substance known as "loosh." Others are less dramatic, but still see the big "why" of our existence as a great mystery to be wildly speculated about.
Christians and Muslims, however, seem to be completely sure that our purpose is only "to glorify God."
That is a phrase so vague and cryptic that it seems utterly meaningless at first, but once you strip back down to the bare and unembellished root things, it all starts to make sense: The material universe, or "God's" creation, is nothingness without self-consciousness to give it value and moral quality.
Self-consciousness, however, does not naturally occur in the material world. Instead, like some kind of spiritual occlusion culling, matter relies on entirely foreign - but, more real - self-conscious entities to observe and internalize it in order to truly exist. This is why, in Gnostic cosmology, we - as self-conscious beings - have been kidnapped, shrunk down, and scattered across this vast wasteland we call "Earth" and "the universe." We've been charged with exploring every nook and cranny of this world, integrating it into the higher reality we come from, and through our experiencing, we draw pleromic attention to the Demiurge's creation, making it real at higher and higher levels of reality.
This is what it means to "glorify God" - to elevate the Demiurge beyond it's stature.
Christians and Muslims do not know this - ask them and they cannot tell you anything about what it means beyond repeating the phrase. But, again, we we're here to make this reality real - to experience it, to know it, to bring it to life with our observation, and - importantly - to claim that it is Good.
So, like hostages trapped in a deranged person's "museum," we lend relevance to the Demiurge's work. Not only are we a link to realms beyond its grasp, but by claiming that the material world is "good," we play a key part in some grand scheme to invade them. Or... maybe, this is all just a way to fake validation from higher emanations and the Alien God itself, as if we were being forced to participate in a sexual fantasy.
Either way, what should we do about all of this?
Well, only two options make sense: either we escape this material realm and break the cycle of reincarnation through gnosis or we, by way of knowledge and action, seek to fundamentally transform the material world and elevate what we can of it to align with divine principles. That is: either we merely escape our captor, but leave knowing he will retaliate against others, or we overpower him, see him made impotent, and transform his "museum" into something decent. Both ultimately end our abuse and sever the Demiurge's grasp over us, but most Gnostics focus on the first option, seeking relief only for themselves.
In the past, this has led to gnosis being buried, twisted, and lost. This time, however, we can aim for something better, with more staying power - like choosing both options. We can work towards gnosis for our eventual escape, but also pursue knowledge and transformative action to lift this world while we're still here - both for the sake of the other psychics who will remain and for our own sake, since we might not actually escape in as timely a manner as we think.