I couldn't get npt.gro file after finishing step npt equlibration step.
First I used this command "gmx grompp -f npt.mdp -c nvt.gro -r nvt.gro -t nvt.cpt -p topol.top -o npt.tpr"
Then used this "gmx mdrun -deffnm"
I got this from http://www.mdtutorials.com/gmx/lysozyme/01_pdb2gmx.html