pamgradient: 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/pamgradient - from package: netpbm-progs

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

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