Final answer:
The secret mentioned is potentially from a literary work, possibly 'A Doll's House' by Henrik Ibsen, where Nora borrows money secretly to save her husband, fearing the truth would harm their relationship.
Step-by-step explanation:
The student is inquiring about a secret shared between characters in a literary work. Without a specific reference to which narrative these characters belong, we will have to make an assumption based on the quotes provided. It seems likely that the secret discussed by Fritz and Gerta could relate to the excerpt from Henrik Ibsen's play 'A Doll's House,' where Nora hides the fact that she had secretly borrowed money to save her husband Torvald's life. She did not tell Torvald because he holds strong opinions against debt and she feared it would damage his manly independence and alter their relationship.
Another possibility is referencing Klaus Fuchs, who admitted to sharing atomic bomb information with the Soviets—a historical event not associated with the characters named Fritz and Gerta. Nonetheless, the concept of secrets and their impacts on relationships is a central theme in many narratives and historical events.