It is very hard to learn much concrete about the gnostic heresy in the first three centuries of the Christian gospel because it was so thoroughly rooted out of the church that it all but ceased to exist, and many of the writings were destroyed.
We know it contained a mixture of greek philosophy and christian ideas and was quite damaging to the faith, causing many to leave or fall away from the faith.
At its core, the duality of gnostic thought devalued this life and moral issues. Since flesh is evil and spirit is pure, we can be spiritual by knowledge and our deeds in this life really do not matter.
It reminds me of the old saying, "so heavenly minded that he is not earthly good", but in a really negative way, not just ignoring the world but indulging the flesh with no sense of consequence since its all about the next life.
Not good, while Christianity placed a great deal of emphasis on the next life and eternal life, it never divorced or split this life from the next. We are making a track record of faith and good works, as a response to grace, and a witness to the world.
This was my friend Alex, who loved living in this world, and left it early, in his 20s. I still miss him.