The Benedict mob gunsel was a gangster who was part of the mob run by Arnie "Eggs" Benedict in Chicago, Illinois in 1927. He fired a tommy gun down an alley at a rival gunsel, with Marty and Doc caught in the middle.
After Marty and Doc talked to Arnie "Eggs" Benedict, they learned that the two gangs were in a feud with each other. Benedict hired Marty to help his gang, as Doc had told him that Marty was the head of another gang from the east coast.