From: HISTORY OF SAN JOAQUIN COUNTY,1923 page 496:
George Willis settled in Stockton in 1869. He was chief engineer for the Wagner Leather Company. Was born in Durham County, England April 5, 1840.In 1867 he left England for America locating first in Ohio, then Omaha, Nebraska.
Mr. Willis' first marriage united him with Miss Hannah Sutherland, a native of England, who passed away in Stockton.His second marriage united him with Miss Mary Blackburn, who was brought to Stockton by Mr. Willis when she was eight years old and received her education in that city.Three children were born to them: Evelhyn, now Mrs. Harry Swift; Hattie is wife of John McAdams, and George B.
I know you're more interested in Robert Gray...but I'm sorry that I can't find any more info on him.