GrADS and GRIB2

 
wgrib : for GRIB1 
wgrib2 : for GRIB2

 
Relationship (Data descriptor : *.ctl files) between GRIB2 and GrADS: 
# ctl name don't matter, but better use cmd below.
$ g2ctl.pl gfs.t00z.pgrb2.0p25.f000 > abc.ctl
$ g2ctl.pl gfs.t00z.pgrb2.0p25.f000 > gfs.t00z.pgrb2.0p25.f000.ctl

 

Explanation of "PRES 0 1, 102,0 Pressure[Pa]" :
1) space deliminator, so it's four parts: PRES / 0 / 1,102,0 / Presssure[Pa]
2)
PRES: name of parameter
0 : 1 level on surface
1,102,0 : parameter description, see below ( VV, LTYPE, LEVEL,TRI)
Pressure[Pa]: unit

Eg of VV, LTYPE, (LEVEL), (TRI):
1, 102, 0 n/a
33, 100 n/a n/a
34, 100 n/a n/a

VV:
1 : pressure
33: wind u component
34: wind v component
LTYPE:
100 : pressure level
102 : mean sea level
(LEVEL)
0 : mean sea level

(TRI)
n/a

Explanation: 
1, 102, 0 : mean sea level pressure

 
Index file (.idx) : tell where fields in GRIB2 are located.
$ gribmap -v -i gfs.t00z.pgrb2.0p25.f000.ctl > grib2_field_vs_grads_var

Note : 1) binary file "gfs.t00z.pgrb2.0p25.f000.idx" will be generated. 
2) Input GRIB2 file "gfs.t00z.pgrb2.0p25.f000" must be present.