How to save the force on a molecule in GROMACS?
Gro output format does not support forces, so using it as an intermediate output format defeats your purpose.

Use

gmx traj -f your.trr -of


which runs the gmx wrapper binary, calling the traj module to do trajectory processing, names your input trajectory file after -f and triggers the output of forces with -of (optionally named after -of, defaulting to force.xvg), or similarly

gmx dump -f your.trr


to dump the entire contents of the file including forces) to the terminal.

See also

gmx help traj


or

gmx traj -h


or http://manual.gromacs.org/documentation/5.1/onlinehelp/gmx-traj.html

Note that I currently develop GROMACS.