gpk-backend-status: 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-backend-status - from package: gnome-packagekit-extra

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

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