South San Francisco police arrested an 18-year-old man late Monday night after he allegedly tried to rob another man at gunpoint, police said today.The 43-year-old victim arrived at his home in the 200 block of Ninth Lane at about 10:45 p.m. and was parking his car when he was approached by a man police later identified as Gonzalo Avalos. Police said Avalos pointed a handgun at the victim and demanded money. At some point Avalos ran away without stealing anything but was quickly apprehended by a police officer. He was arrested and booked into jail on suspicion of armed robbery, police said.The victim was not injured. Officers searched the area but did not find the gun. Anyone who has information about the attempted robbery, or who finds the handgun, is asked to call the South San Francisco Police Department at (650) 877-8900.