gufw: 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/gufw - from package: gufw

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

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