Jacob born March 1831 in Darmstadt, Germany married Katherina. I do not know her maiden name, all records I have list her as Katherina Gillen so she may have been a Gillen too. She also was born in Darmstadt. They immigrated to the U.S.A. in 1850 and settled in Manhattan and Brooklyn, New York.
They had 5 children all born in New York. George William, born Oct. 1852, Catharina born May 13, 1856, Frederick Jacob born Feb. 28, 1859, Gustav born 1862 and and unknown child.
I would like to know who Jacob and Katherina's parents were and if they had any siblings.
I have done some research on a site called Italian Genealogy Group, don't let the Italian in the name throw you, This site has the indexes of New York City Births. deaths, marriages and nationalizations. I found only one record for the death of a Jacob Gillen, this died in 1903 at the age of 65 , certificate no. i 21912. You can order the death certificate from the City Archives. The parents names are listed on the death certificate, that is how I found the names of my great great grandparents. It might also contain Katherina's maiden name.
Here is a link to this site there are plenty of Gillen's listed. http://www.italiangen.org