Helen, a tip of my hat to you. I have always associated 'gussied up' with 'gusset'. A gusset may be used for other purposes than in clothing. It may be quite a large piece used to extend or widen something; e.g., theatrical scenery.

What comes to my mind when I hear the term is those incredible Victorian ladies' dresses with the rows of ruching, flouncing, darts and -- here it comes -- let-in panels, or gussets.