Gloucester's misfortune is to misled by his son Edmund. Like Lear, Gloucester is blind to the real natures of his children. Edmund, his illegitimate son, is a ruthless villain who is plotting against his father, Lear, and Lear's daughters. Edgar, his legitimate son, is loyal to Gloucester, but Edmund tricks Gloucester into thinking that Edgar is plotting against him.
Gloucester's turn against Edgar parallels Lear's banishment of Cordelia. Both men are unable to recognize true affection, in part because of their vanity. Gloucester has a high opinion of his own cleverness and political savvy, which makes it easy for Edmund to exploit him. It is only when Gloucester's eyes are gouged out by Cornwall that Gloucester can truly "see" the truth about his sons.
The problem of "vision" or recognition is a major theme in the play. Gloucester is unable to recognize Edgar in his disguise as Tom o'Bedlam in the same way that he is unable to see where the loyalties of his children truly lie. After Gloucester's eyes are gouged out, and he is left to "smell" his way to Dover, Gloucester comes to realize how mistaken he was about his children and about himself. His final reconciliation with Edgar, while gratifying, comes too late to save him.