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

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

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