Gladstone's Luck, also known as Lady Luck, is a mysterious entity or force of unknown nature.
Throughout their lives, Daphne Duck and her son Gladstone Gander were and are (respectively) the wards of a supernatural luck. This luck appears to be somewhat sentient, making its own creative decisions in how it manipulates probability around its ward(s) and starting to work against Gladstone if he ever appears to be using his abilities for evil.
According to a source which accounts for both Daphne and Gladstone's supernatural luck, this good fortune was apparently to their beings by a triple-distelfink hex sign painted on the day of Daphne's birth by a mysterious Traveling Painter. As Gladstone was born on the same day as his mother, he inherited the luck granted by the hex sign. Other accounts give different sources for Gladstone's luck. Indeed, Gladstone once indicated that his wishing coin was the cause of his good fortune, and a magical being known as "The Lepra-Duck" once declared that Gladstone's luck was derived from the Lepra-Duck's own wishing stone, which he briefly stole from Gladstone in 1966 and temporarily gave to Gladstone's cousin, Donald Duck.
Behind the scenes
Gladstone's Luck was created at a plot device in his fourth appearance, Race to the South Seas, in 1949. Some stories define it as something akin to a living, conscious being, while others leave it as an ambiguous force.