Fatal Error: Residue 'DPP' not found in residue topology database

GROMACS version: 2020.1
GROMACS modification: No
Hi All,
I am currently working on a project to use GROMACS as a way to explore what factors affect liposome fusion and separation. I generated a liposome pdb file using packmol. I then used insert-molecules to generate a new pdb file with two liposomes, and was planning to run gmx pdb2gmx to get a topology file. I got the above error, Fatal Error: Residue ‘DPP’ not found in residue topology database, and am unsure of how to proceed. Looking on the GROMACS website it appears that this error is due to the amino acids not matching the database GROMACS has, however there are no proteins in my system so I am unsure of why it is trying to match amino acids. I am generally unsure of how to proceed with the project and any tips or guidance on how I could use GROMACS to accomplish this goal would be greatly appreciated.