This wiki page
https://en.wikipedia.org/wiki/NATO_Stock_Number
has some info on deciphering the FSN/NSN/NIIN codes found on packaging as well as a little history about when certain designations came into use - e.g., FSN >> 1953 to 1974, NSN >> 1974 to present
The GS numbers are numbers assigned by the GSA to products purchased from commercial sources to be made available to the governmental organizations (DoD, USPS, USDA, etc) that use the NSN system to purchase supplies. These GSA purchased items may only be available in the system for a short time. Assigning "real" NSNs to them would create a logistical nightmare. The way the GS handles these "temporary products" is to create the GS system for ordering these items.
Take the Camillus made pilot's knife above -
The date 10/83 tells us it is after Camillus lost the contract for producing these for the NSN system. People still wanted them, Camillus still had them in stock and could make more, and wanted to sell them through the PX/BX/AAFES system. So the GSA assigned a GS number to an item that just a few months before would have been sporting an NSN. In fact, in this example, the only thing Camillus had to do was change the NSN line on the bag printer to the GS number.
The GSA simply adds a GS followed by an Item Category Code to the Company Part Number of the item in question.
The GS tells us it is a GS product, available through the GSA catalog system. The "09S" is the category type for "knife". The 42296 was Camillus' own part number for the knife.