g_make_ndx: 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_make_ndx - from package: gromacs

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

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