gnugk: 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 Debian possible default full paths are:

/usr/sbin/gnugk - from package: gnugk

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

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