The word "gaud" derives from the Latin word gaudēre, to rejoice, and it originally referred to any of the fifteen beads on a rosary, a counting device used by Catholics for prayer. As rosary beads became more ornate, the word "gaud" came to refer to any ornate or brightly colored trinket, and that's how Shakespeare's using it here. Today, we use the adjective "gaudy" in a negative sense to describe something brightly colored and ornate, but lacking in taste.