if I am correct you are referring to the old style of package names in Gromacs, i.e. g_[tool]. In that case, g_hbond and gmx hbond both refer to the same tool – gmx hbond is just the new way to launch it.
Note that if they are different versions the two may not be identical. You can check how it defines the bond by running gmx hbond -h, which lists the default settings for bond distance and angle.