Can one say that Mandeans are gnostics, or gnostic Christians?
Yes, Mandaeans are gnostics, but not gnostic Christian, Mandaean faith is very different with Christianity. Mandaeans are under influence of Pythagorian, Egyptian and Akkadian philosophy and astrology. Manichaeism is also under influence of Mandaeans. Father of Mani, Patek or Fatek, was a Mandaean. Mani himself was Mandaean before that he thought his new faith.