Depends on which part of Germany you are looking for...
I've been tracing my Grunewald family history from Rheinland-Pfalz (Bavaria). The family I've been tracing lived in and around Kaiserslautern, Germany since the 1700's.
What is the name of your grandfather?
I, along with several distant cousins, have a fairly large Grunewald (Gruenewald, Greenwald, Greenwalt) tree mapped out with Civil Registration documents to back it all up.
Many of the Greenwalds or Greenwalts that you see in the US came from the Grunewalds. Have you tried looking for ship logs when your relatives came into the United States? This document may have a clue in which city your family came from.