The gain_scale correction step uses the gain reference file. The only purpose
of the reference file is to retrieve the
GAINFACT keyword value from its
header (the reference file data are not used in any way). If the
step, which also uses the gain reference file, succeeded in finding the
GAINFACT keyword in this reference file, it will store the value in the
GAINFACT keyword in the science data, in which case the
will not reload the gain reference file.