On May 1, 2007 12Z, the GFS had a major change. The vertical coordinate became a hybrid sigma-pressure coordinate instead of a pure sigma coordinate. This change caused the internal binary GFS history file to change formats. The file is still in spectral space but now pressure is calculated in a different way.
Sometime in the future, the GFS history file may be more independent of internal model configuration. But for now, users of the GFS history file must be very flexible and make extensive changes when the format changes like this. That is, users of the raw GFS history file must realize all risks in its use.
The links below are aids to using the GFS history file. The first link is to a MSWORD document intended for NCEP internal users in the conversion to the new format. Many of the options given there will not be available to outside users. However, its contents may be of use in conversion. The second link is to the "sigio" Fortran90 module that is used in NCEP to read the history file. It should be of much use in reading the file.