GROMACS for non proteins?

GROMACS version: 2018.3
GROMACS modification: No

I’m completely new to this but is GROMACS usable if I am trying to model the self-assembly of three molecules (one long-chain omega hydroxy fatty acid, glycerol, one phenolic acid) in a given solution? No proteins at all. I am following the Lysozyme tutorial (force field OPLS-AA/L, water TIP4P TIP 4-point) but get fatal error after running

gmx pdb2gmx -f file.gro -p -ignh

Fatal error:
Residue ‘C’ not found in residue topology database

and after grompp this error:

Fatal error:
number of coordinates in coordinate file (fgjc1_solvv.gro, 3149) does not match topology (, 40104)