gp: 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/gp - from package: pari-gp

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

On Debian possible default full paths are:

/usr/bin/gp - from package: pari-gp

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

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