Can gusts be reported as lower than wind speed?
WMO standard defines a gust as the maximum wind speed exceeding the &quot;mean speed&quot; by 5 m/s (10 knots) during the 10-minute interval. With this definition, I would never expect to see a negative gust factor.
However, the FAA defines it differently (FMH-1):

5.4.3 Wind Speed.
The wind speed shall be determined by averaging the speed over a 2-minute period.
5.4.4 Wind Gust and Squall.
The wind speed data for the most recent 10 minutes shall be examined to evaluate the
occurrence of gusts or squalls. Gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knots or more between peaks and lulls.

Thus, by my interpretation, a U.S. airport METAR can report gust speeds below the mean reported wind speed since it only requires a variation of 10 knots, not an exceedance of 10 knots.
However, reading further in the same document:

5.6 Summary of Wind Observing and Reporting Standards
Wind speed: 2-minute average speed in knots is reported.
Wind gust: The maximum instantaneous speed in knots in the past 10 minutes is reported.


It seems impossible to have a 2-minute average speed which is higher than the maximum instantaneous speed over the past 10 minutes. So following that, it would seem to indicate an error in the measuring or reporting equipment.