g_mk_angndx: 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_mk_angndx - from package: gromacs

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

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