A draft description of the GROMOS force field by the IGC group at ETH, Zurich is given here. The literature reference for the GROMOS 53A6 force field is here.

GROMOS 54A7 IFP files with additions from the ATB
GROMOS11 54a7 ATB.ifp	GROMOS96 ifp54a7 ATB.dat

Additional GROMOS forcefield files can be found here.

Original GROMOS Force Field files for use with GROMACS
GROMOS_54A7_for_Gromacs_3.x_4.0.x.tar.gz	GROMOS_54A7_for_Gromacs_4.5.x.tar.gz

Modified GROMOS Force Field files for use with GROMACS including non-standard or modified atom type parameters
GROMOS_54A7_for_Gromacs_3.x_4.0.x.tar.gz	GROMOS_54A7_for_Gromacs_4.5.x_5.x.x.tar.gz

GROMACS v-site bug fix for S-H: GROMOS_54A7_for_Gromacs_3.x_4.0.x_HS.tgz

Preview - Manual (Vol. 2) - General Algorithms and Formulae
Preview - Manual (Vol. 3) - Force Field and Topology Data Set
Preview - Manual (Vol. 4) - Data Structures and Formats
Preview - Manual (Vol. 5) - Program Library Manual
Preview - Manual (Vol. 6) - GROMOS Technical Details