The phrase "gas molecules" refers to tiny particles that make up gases. They are like very small building blocks that float around and move freely in the air. Full definition
Surely the ideal gas molecule random movements are consistent with irregular motion. (wattsupwiththat.com)
The satellites do not measure temperature directly, but instead observe the intensity of microwave radiation from gas molecules in the atmosphere. (corporateknights.com)
Each comb «tooth» represents an individual color, or frequency, enabling very accurate measurements of the characteristic absorption signatures of different gas molecules of interest. (sciencedaily.com)