g_nmens: 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 CentOS possible default full paths are:

/usr/bin/g_nmens - from package: gromacs

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/g_nmens - from package: gromacs

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

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