g_cluster: 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_cluster - from package: gromacs

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

On Debian possible default full paths are:

/usr/bin/g_cluster - from package: gromacs

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

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