What is my great uncle' son to me?

There are several possibilities, including "no relation" (or at least none that can be described more concisely than "great uncle's niece").

Assuming the maximum possible degree of consanguinity, your great uncle's niece could be your mother. She could also be an aunt (your mother's or father's sister, depending on which side the great uncle is on), or as mentioned she could be "no relation" (if the great uncle is an uncle by marriage and his niece is the daughter of one of his brothers or sisters).