Gascon has been the police chief of Mesa, Arizona for three years.
The current SF police chief, Heather Fong is leaving the post.
Gascon, 55, will be the first outsider to head the San Francisco Police Department in 34 years.
Newsom said hiring from outside the department is a risk. But he expressed support for Gascon, calling him a "cop's cop."
Gascon emigrated from Cuba as a boy, is an advocate for sanctuary policies for illegal immigrants, and is an expert on the issue of racial profiling of suspects.
None of the three finalists whose names were forwarded to the mayor by the Police Commission last week was from within the department.