This means that the livestock being sold has been tested and found to be free of certain specific diseases or conditions. The specific diseases or conditions that are guaranteed negative will be clearly stated in the sale contract or documentation.
For example, a sale might be advertised as "GNS for BVD, IBR, and PI3." This means the livestock has been tested and found to be free of Bovine Viral Diarrhea (BVD), Infectious Bovine Rhinotracheitis (IBR), and Parainfluenza 3 (PI3).
The GNS designation provides buyers with assurance that the livestock they are purchasing are healthy and free from certain diseases, which can be important for herd health, production, and overall profitability.