What constitutes a wind gust in a METAR report?
The definition of "Wind Gust" for METAR reports can be found in the Federal Meteorological Handbook No. 1, chapter 5:


  5.4.4 Wind Gust. The wind speed data for the most recent 10 minutes shall be examined to evaluate the
  occurrence of gusts. Gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knots
  or more between peaks and lulls. The speed of a gust shall be the maximum instantaneous wind speed.