nntpgrab_server_gtk: command not found

Have You tried accessing the binary with the full path?

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/nntpgrab_server_gtk - from package: nntpgrab-server-gtk

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