What's wrong with believing in gnosticism?

(I know, He)
This is the trouble. People jump past the meaning of words. It should be a simple answer..... You know? (that's a hint)

Gnostics believe that salvation comes through knowledge, not through the saving action of Christ and the belief and works that accompany following Him. It's based on a false Greek notion going back to Socrates which holds that knowledge of the good leads to performance of the good, which is clearly false. The world is replete with examples of people who have known what the good action to perform is in a situation, but we fail to do it or worse yet we commit the evil choice instead.

Believe whatever you want to as it is all a crock.

In whose eyes is it wrong? There is nothing at all wrong with believing in gnosticism. Wrongness is in the mind anyway - it's a subjective thing.

Nothing - whatever tickles your fancy - I am of the opinion that all religion, like politics, should be banned as it causes so much trouble! Especially as we are only speculating in reality!!! There is no real proof to support any religion!

You have answered your question....you cannot believe in gnosticism because gnosticism does not require belief.

Ancient Gnostics (let us put aside the modern variants for now) focused on understanding the relation of man to the divine. We can only guess at their full story, since most of their records have been destroyed. Nevertheless, for a gnostic, salvation appears to have been a process that required intellectual, emotional, and physical development.

The modern Catholic Church considers gnosticism to be the greatest heretical enemy it ever faced. In fact, each verse of the Nicene Creed (the statement of belief still said in every Catholic, Orthodox, and Anglican mass) is worded specifically to combat it. The Church considered association with Gnosticism wrong for one reason...all Gnostic's state that the world is not good nor created by a good creator. This idea undermines the entirety of Christian thought.

Is the idea wrong objectively, though? That is another question. I will only suggest that the answer is not simple. Moreover, you will find few texts, still existing, that address it without a knee-jerk reaction to one side or the other.