Where does the saying the gilt has worn off the gingerbread come from?

That phrase is not talking about the gingerbread you eat! Victorian Era houses had a lot of decorative woodwork that was called "gingerbread" because it looked a bit like the decorative gingerbread houses people make. If your gingerbread on your house had gold leaf rubbed on, it was gilded, or gilt. "The gilt has worn off the gingerbread" thus means that time has passed and the decorations aren't as pretty - in other Words, the newness has worn off.