The NCEP website describes the file naming format of the GFS forecast data.
What does "forecast hour of product" mean?
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The forecast hour of the product is the hour after the model cycle runtime that the forecast is valid at. For example, if the the cycle runtime is 12Z and the forecast hour is 9, then the the forecast is valid for 21Z. If the cycle runtime is 6Z and the forecast hour is 48, then the forecast is valid at 6Z two days later.