A 101-year-old man died in a fire at a eight-unit residential building in the Rossmoor older-adult community in Walnut Creek on Saturday, the Contra Costa County coroner's office said Monday. Walnut Creek resident Arthur Gertz died in the fire reported at 6:21 p.m. Saturday in the 1200 block of Leisure Lane. Gertz died in the only unit that burned in the fire. Three other units had smoke and water damage and residents in all four units were displaced, Contra Costa County Fire Protection District Fire Marshal Chris Bachman said. The cause of the deadly blaze remains under investigation as of Monday, fire officials said.