gmx: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/gmx - from package: gromacs

You may install the required package with command apt-get install {package}

Error in other languages:
gmx: bevel nie gevind nie
gmx: comandă negăsită
gmx: comando não encontrado
gmx: commande introuvable
gmx: command not found
gmx: käsku ei ole
gmx: không tìm thấy lệnh
gmx: komanda nerasta
gmx: Kommando nicht gefunden.
gmx: kommandot finns inte
gmx: komut yok
gmx: nie znaleziono polecenia
gmx: níor aimsíodh an t-ordú
gmx: no se encontró la orden
gmx: no s'ha trobat l'ordre
gmx: opdracht niet gevonden
gmx: parancs nem található
gmx: perintah tidak ditemukan
gmx: príkaz nenájdený
gmx: příkaz nenalezen
gmx: команда не найдена
gmx: командата не е открита
gmx: 命令找不到
gmx: コマンドが見つかりません