trj file format
Main Table of Contents VERSION 5.0.5

Description

Files with the trj file extension contain the trajectory of a simulation. In this file all the coordinates, velocities, forces and energies are printed as you told GROMACS in your mdp file. This file is in binary format an can be read with gmx dump. 
% gmx dump -f traj.trj
or if you're not such a fast reader: 
% gmxdump -f traj.trj | more

You can also get a quick look in the contents of the file (number of frames etc.) using: 

% gmx check -f traj.trj

http://www.gromacs.org