In the novel A Caribbean Mystery, Gail Dyson was the first wife of Greg Dyson. She died four years before the events of the novel, in Martinique, leaving Greg a lot of money. Lucky was a cousin of hers, and had gone out with her and Greg. Gail was an invalid, and Lucky gave her all her medicines.
It is later revealed that Lucky poisoned Gail, saying that Gail was suffering, and had begged her to get something that would end it all. After Gail's death, Lucky married Greg.