gprolog: 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 CentOS possible default full paths are:

/usr/bin/gprolog - from package: gprolog
/usr/lib64/gprolog-1.3.1/bin/gprolog - from package: gprolog

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/lib/gprolog-iso/bin/gprolog - from package: gprolog

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

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