saga_gui: command not found

Have You tried accessing the binary with the full path?

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/saga_gui - from package: saga

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