gpk-repo: 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-repo - from package: gnome-packagekit

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

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