Gabbe surname

My major roadblock in research is discovering the history behind the man with this surname. His name was Emil Arthur Gabbe born in Berlin and immigrated at the age of 16 via Hamburg to the United States. He landed in New York in 1890, then from there went to Illinois then on to California.  His life story from the the age of 16 till his death are no mystery.  The roadblock, is finding out his past and his family in Germany.  His mother was Mathilde, his father was August. I have a letter from his sister Louise married name "Herner" from 1917 at that time her parents were living in Velten, Germany.  Does anyone know of this family or how to go about researching it, without traveling to Germany?