Final answer:
The letters GAS on a NATO marker indicate the presence of chemical hazards such as toxic nerve agents, blister agents, blood agents, or choking agents.
Step-by-step explanation:
The letters GAS on a standard NATO marker represent chemical hazards, specifically indicating the presence of toxic agents that may be harmful or lethal if inhaled, ingested, or come into contact with skin. These markers would be used in a scenario where chemical weapons have been employed to warn individuals, including military personnel and civilians, of dangerous substances in the area. In this context, the GAS marker would typically be used to denote areas contaminated with nerve agents, blister agents, blood agents, or choking agents.