Refers to birthday gifts shipped in bulk. Gang, an organization that transports goods in batches, such as tea gang, salt gang, and flower gang. The second definition of "gang" in "Dictionary of Commonly Used Ancient Chinese Words": the organization of goods transported in batches during the Tang and Song Dynasties. Such as "Yan Gang", "Cha Gang", "Huashi Gang", etc.
For example, in "Water Margin·Chapter 16" Yang Zhi escorted gold and silver...