gpk-prefs: 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-prefs - from package: gnome-packagekit

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

On Debian possible default full paths are:

/usr/bin/gpk-prefs - from package: gnome-packagekit-session

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

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