It's hard to generalize a surname's origins. However, we can safely say that Gerwitz is European in origin. A good many of the Gerwitzes in the world today descend from the Gerwitz family which lived in the Baden region of Germany near a town called Onsbach-Achern in the SW region.
However, there are Gerwitzes today living in England, Australia, the US, and other parts of the world who do not trace their ancestry back to the Gerwitz family from Onsbach. Instead, their ancestors came from Russia, Prussia, Poland, the Czech Republic and even other cities in Germany.
In addition, there are many related names such as Gurevich, Goerwitz, Horowitz, etc. all depending on which country you are talking about.
It is speculated that the Gerwitzes of Onsbach originated in Eastern Europe and are of Jewish ancestry, but this cannot be proven. Records before 1700 are scarce and anyone who wished to hide his Jewish status by moving to a new country, adopting a new spelling of one's name, or even changing one's faith would not purposely leave behind records of this action.
It would be quite interesting to connect all of the Gerwitzes in the world today to one ancestor, but this task will probably prove impossible despite the uniqueness of the name.