It's the maximum gust in the 10 minutes up to and including the observation time. So, in this case, the observation is nominally for 20:00 UTC on the 11th, so the actual time of the observation was 19:50 UTC. Therefore the period of the maximum gust in the 910 period in this case is 19:40-19:50. It's the same gust you would see in a METAR - the one after the "G" is, for example, 25015G25KT.