Monday's robbery occurred after 9 p.m. near the intersection of Euclid Street and Hazard Avenue, where the robber entered the truck and brandished a semi-automatic handgun, Garden Grove police said in a statement.
The gunman demanded money from the employee, who turned over about $250, officials said. The robber then fled scene in a silver Jeep Cherokee.
He was described as black, 40-45 years old, 6 feet tall and about 250-300 pounds. He was last seen wearing a sleeveless white T-shirt and oversized gray shorts.
Anyone with information about the crime is asked to call the Garden Grove Police Department at (714) 741-5704.