gprolog-iso: 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/gprolog-iso - from package: gprolog

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

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