gpk-service-pack: 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/gpk-service-pack - from package: gnome-packagekit-extra

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

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