I don't know if Johann Niklas(1744-1820) had any other children than John Emmanuel(1799-1877). We haven't tried to research that at all. However, in the 1841 English census there is a John Ganson, not born in England, of appropriate age, living in Droylsden with seven children. His first wife, Susanna, had died in childbirth in 1838, according to the Moravian records, so she wouldn't be in this census. One of the seven children is a Frederic, born in 1824. The other children listed are Edwin, William, Augustus, Emma, Eliza, and Louisa. Another child, Nicholas, was stillborn or died shortly after birth along with his mother, in 1838. Johann Niklaus, according to the Moravian records, died in 1820 so this Frederic couldn't be his child.
We would be interested of any information you have about any additional children of Johann Niklas. We would also be interested why you believe that John Emmanuel did not have a son called Frederic(k).