Hi there!
When Gertrude says “fear me not”, she indicates that she was not involved in King Hamlet’s murder.
Gertrude was Hamlet’s mother and also, Queen of Denmark. After her husband’s death, King Hamlet –father of her son, Hamlet-, she married King Hamlet’s bother, Claudius after he murdered the King. Her son, Hamlet wasn’t happy with his mother’s choice. According to Hamlet, Gertrude scarcely mourned her husband’s death before marrying Claudius. By saying that line, Gertrude is telling Hamlet to not see her as an enemy.
Hope this helps!