apt-offline-gui: 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/apt-offline-gui - from package: apt-offline-gui

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

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