The best source for info on decoding US METARs is the
Federal Meteorological Handbook 1 (FMH-1).
As to your question, I can't find anything for GF. S is South, LAST indicates it's the last observation of the day for a non-24h station.
Wasn't GF the indicator for ground fog in the old SA sode? Maybe the observer is trying to indicate
Ground
Fog
South of the station.