gdnc: 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/gdnc - from package: gnustep-base

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

On Debian possible default full paths are:

/usr/bin/gdnc - from package: gnustep-base-runtime

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

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