gp-2.3: command not found

Have You tried accessing the binary with the full path?

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/gp-2.3 - from package: pari-gp

You may install the required package with command yum install {package}
Error in other languages:
gp-2.3: bevel nie gevind nie
gp-2.3: comandă negăsită
gp-2.3: comando não encontrado
gp-2.3: commande introuvable
gp-2.3: command not found
gp-2.3: käsku ei ole
gp-2.3: không tìm thấy lệnh
gp-2.3: komanda nerasta
gp-2.3: Kommando nicht gefunden.
gp-2.3: kommandot finns inte
gp-2.3: komut yok
gp-2.3: nie znaleziono polecenia
gp-2.3: níor aimsíodh an t-ordú
gp-2.3: no se encontró la orden
gp-2.3: no s'ha trobat l'ordre
gp-2.3: opdracht niet gevonden
gp-2.3: parancs nem található
gp-2.3: perintah tidak ditemukan
gp-2.3: príkaz nenájdený
gp-2.3: příkaz nenalezen
gp-2.3: команда не найдена
gp-2.3: командата не е открита
gp-2.3: 命令找不到
gp-2.3: コマンドが見つかりません