You can use comm-grps and comm-mode options in your mdp file. For details, search the two options here:

http://manual.gromacs.org/online/mdp_opt.html#run

GROMACS does not have the ability to restrain a molecule based on its center of mass. The refcoord-scaling option determines how the origin of the harmonic position restraints is calculated, either scaled based on a relative (internal) coordinate calculated as the vector originating at the COM, or by each atom's position individually (refcoord-scaling=all). This is dealing only with how the reference positions are updated when pressure coupling is applied.

Gromacs isn't capable of that.

In Lammps though, it is possible with the fix recenter. If you are not too deep in Gromacs, then you might consider switching to Lammps. I must warn you that Lammps has a bit of learning curve compared to Gromacs though. If restraining is not necessary, then you can stick to Gromacs.