gnt-instance: 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/gnt-instance - from package: ganeti

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

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