Gas-pak is a method used in production of an anaerobic environment. It is used to culture bacteria which die or fail to grow in presence of oxygen (anaerobes). It is a much simpler technique than the McIntosh and Filde's anaerobic jar where one needs to pump gases in and out.
There are commercially available, use-and-throw sachets containing a dry powder or pellets, which when mixed with water and kept in an appropriately sized airtight jar, produce an atmosphere free of elemental oxygen gas. These are used to produce an anaerobic culture in microbiology.
Read more about Gas-pak: Constituents of Gas-pak Sachets, Reactions, Consumption of Oxygen, Culture Method