Does gnosticism hold that matter is evil? This seems to be the view of one particular writer... that even the beauty in the world, such as a sunrise or a rose, is a deceptive, thin patina that the demiurge uses to cover over the ugly reality of this world.The flat statement "matter is evil" comes from the heresiologists not the Gnostics and is intended to be a polemical statement. There are differences in the way that various Gnostics viewed matter from the writings that we have. In some they do seem to view it as a very bad if not terrible condition to be in or a part of. This is different than viewing it in the polemical dualism of matter equals evil, the immaterial (spirit) equals good.
Do you think that the aim is to realize gnosis and then leave this cosmos entirely (when we die), or do you think that the purpose is to realize gnosis and thus bring the light and love of the pleroma into this cosmos to transform it? In other words can this reality be redeemed or is it so inherently flawed that this is not possible and that the only way is out (so to speak).
The majority view from Gnostic texts seems to be more of equating matter and material things with ignorance, or lack of consciousness, or distance from the divine. It is not a black or white or binary understanding, but rather a continuum—a more spiritually-oriented condition being far better than a more materially-oriented one. I really have a hard time with the notion that ancient Gnostics were all that concerned about matter itself, or of the physical world playing such an important role for them in and of itself. In scriptures it is more one's attitudes, goals, consciousness, participation, and most importantly Gnosis that are the concern. Liberation does not requires the end of material existence, but rather individual transformation through Gnosis.
To only judge the ugly as real and so judge the beautiful to be an illusion seems as big of a mistake as doing the opposite. Beauty occurs. It can be found even in the midst of the darkest tragedy, and also just where you expect it to be. It is better to simply see what is in front of our faces, as Jesus says in Thomas, than try to make it into evidence of some kind. An aesthetic experience is simply an aesthetic experience. We are always a part of such experiences whether they take place as a result of human action or as a result of natural action. Is there a need for a creative artist of nature? The photographer creates art by seeing it and capturing what is seen. We as conscious beings are able to see and experience deeply. In Jung's view, consciousness is the means by which all that is comes at last to know itself. When we experience beauty we are not mere observers, we are participants.
The way that seems to sum up the Gnostic attitude is viewing the cosmos as a mixture or mixing of spirit and matter or of light and darkness. In this realm the sparks of the divine are within the limitations of matter and the danger is that they remain in ignorance of that divine reality and their connection to it. The precosmic creation stories tell how this situation came to be, and the salvation stories tell of how this ignorance can be remedied.
Your articulation of bringing light into the cosmos is correct, and we are to do this in our lives here and now. There is no indication in any of the texts, and nothing from modern experience to suggest that a spirit attaining liberation upon death would return to the Fullness and then come back into the cosmos. My thought about it is that any return would not be possible, being something like trying to fit your grown self into the same crib you slept in when first born. Part of the specialness of Christ is his being able to make the sacrifice to come into the limitations of matter for our sakes.
However, there is good evidence from scripture and modern experience that we as Gnostics are to bring light into the cosmos through our own growth in Gnosis. The resurrection is an inner spiritual event that occurs in this life, and all of the mysteries including the redemption/liberation and the bridal chamber are intended to be accomplished here. This is our highest task as Gnostics. The awakening of an individual is a cosmic event. Within the person of light there is a light that illumines the cosmos, as Jesus says in the Gospel of Thomas.
However, there is no indication that I am aware of where this leads to some complete transformation of the cosmos. Ultimately, the cosmos is a system with the limitations of a system. Some of those limitations are the nature of time itself, and the constant increase of entropy. To have the cosmos be otherwise would mean it isn't the cosmos or wouldn't be the same place we live now. It is something like wishing someone you love had no flaws and no limitations and so on, so that they would not be the same person that you love anymore. The cosmos is in so many ways simply different than the Fullness. But I think that it too will continue in some way having had its interaction with the spirit here and now, that coming to a fuller consciousness here means that we will return something of it as a part of ourselves when we return. It is like the Hymn of the Pearl, there is something that has been transformed here that will be returned with us. The transformation itself may be thought of as the treasure sought after here and returned with us to our true home.