Not just hg and gdg gg. history of GG exp is very vast. it started during 70's/80's as SUR-SBC MG GG exp
sur dep 9:15 pm, bjp 00:00, ubl - 7:00. and from ubl it was actual ICE during mg days, current days siddanganga ICE. i still remember it used to reach sbc around 6 pm. sbc dep 11:30 am next day ubl 10 pm, bjp 4:00 am, sur 6:30. after 1994, this discontinued from gdg to sbc and started as sur - gdg gg exp. then during Y2K. it started as bjp-gdg gg exp. later came the current days BG version from 2008/9