These are some pdb and rtp files used in creating molecular topologies for use in GROMACS. It's recommended that you first copy your GROMACS forcefield directory before modifying anything and then setting the GMXLIB environmental variable to the new location.
To create a molecular topology simply copy all of the rtp files to your GMXLIB's oplsaa.ff folder. Then run pdb2gmx on the molecule in which you are interested. For example, for methane run:
gmx pdb2gmx -f methane.pdb
Be sure to choose OPLS obviously.
I've tried to make sure that the output topologies are correct and don't have anything missing. However, it is your job to ensure that they are correct and there is no warranty that they are correct. Some of the pdb files depend upon multiple rtp files, which is why it's a good idea to copy all of them over.