g_luck: 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_luck - from package: gromacs

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

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