gretl: 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 - from package: gretl

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

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