What you see in the top file are just the indexes corresponding to the atoms involved in a given dihedral. For finding the torsion angle values themselves you have to look up your force field parameters. These are part of your topology due to an "include" statement. If you go to the folder /your_path_to_gromacs/share/top you'll find the "ffbonded.itp" file with the torsion parameters you're asking for.

Althought you have a default value, you need to specified it in your molecule file. The path, in your case, would be, usually, /usr/local/share/gromacs/top/amber99sb.ff, and the information should be in the file that David said ffbonded.itp, in the dihedraltypes.

You do not need to add as a *.rtp and use the pdb2gmx, you could use it as *.itp and add the file path/file name to the *.top file. It is easier, and you do not modify the force field original version.

Thank you for the answers. I have solved the problem.

I know where to find dihedrals in force field, the question was how to check what values are actually used for MD. It is not the same that to find value - may be I found not proper value. For example, there is dihedral for X-X-CM-CT if the force field, I suppose that it will be used, but grompp says - no default dihedrals found, and I have to specify it manually. May be there is a possibility to extract it from *.tpr file?

In the *.top file you will have all the specification about bonds, angles and dihedrals. I work with Gromos, I don't know if in amber is different or not, but, at least in gromos, you need to specified the dihedrals, as I said it before.