ppp-gatekeeper: 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 Debian possible default full paths are:

/usr/sbin/ppp-gatekeeper - from package: ppp-gatekeeper

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

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