gphelp: 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/gphelp - from package: pari-gp

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

On Debian possible default full paths are:

/usr/bin/gphelp - from package: pari-doc

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

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