gretl_x11: 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/gretl_x11 - from package: gretl

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

Error in other languages:
gretl_x11: bevel nie gevind nie
gretl_x11: comandă negăsită
gretl_x11: comando não encontrado
gretl_x11: commande introuvable
gretl_x11: command not found
gretl_x11: käsku ei ole
gretl_x11: không tìm thấy lệnh
gretl_x11: komanda nerasta
gretl_x11: Kommando nicht gefunden.
gretl_x11: kommandot finns inte
gretl_x11: komut yok
gretl_x11: nie znaleziono polecenia
gretl_x11: níor aimsíodh an t-ordú
gretl_x11: no se encontró la orden
gretl_x11: no s'ha trobat l'ordre
gretl_x11: opdracht niet gevonden
gretl_x11: parancs nem található
gretl_x11: perintah tidak ditemukan
gretl_x11: príkaz nenájdený
gretl_x11: příkaz nenalezen
gretl_x11: команда не найдена
gretl_x11: командата не е открита
gretl_x11: 命令找不到
gretl_x11: コマンドが見つかりません