gksu: 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/bin/gksu - from package: gksu

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

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