I sometimes get some really great answers from my students in the form of essays.

Here's a sample from one of my favorites.  It's by student Mary Grace in my class The Big Picture - Connecting the Dots in Salvation History.

St. Thomas Aquinas states that truth is the mind’s conformity with reality, with “what is.” Gnosticism essentially reverses this by denying objective reality while attempting to make its own truth. More examples of this denial and refusal to conform with reality are the transgender movement, homosexual unions, and abortion. Some members of the Church today want the Church to change her teaching on issues such as these. Gnostics think that they can change reality and that it evolves, however the truths of the Catholic Church are perennial.

UPDATE: Also, I really like this from Laura ... 

Where is it today? Gnosticism still crops up all over the place. One kind of new ‘scientific’ Gnosticism puts God entirely out of the picture. The material universe is the only reality but it is a faulty one. Science must attain control over this matter and transform it; this is why the aim of technology often manifests itself in the manipulation and recreation of human nature. Another new Gnostic is bent on eliminating gender and this has deeply colored our culture. This is seen in the defeminization of women; it is a Gnostic notion that being a wife and a mother is a a burden imposed by biology. Gnostics have always wanted to simplify things. The mysteries God has shown us are too much for the human intellect to grasp, so Gnostics seek to reduce all the complexities of reality to a few simple sentences that anyone could understand