gdm3: 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/gdm3 - from package: gdm3

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

Error in other languages:
gdm3: bevel nie gevind nie
gdm3: comandă negăsită
gdm3: comando não encontrado
gdm3: commande introuvable
gdm3: command not found
gdm3: käsku ei ole
gdm3: không tìm thấy lệnh
gdm3: komanda nerasta
gdm3: Kommando nicht gefunden.
gdm3: kommandot finns inte
gdm3: komut yok
gdm3: nie znaleziono polecenia
gdm3: níor aimsíodh an t-ordú
gdm3: no se encontró la orden
gdm3: no s'ha trobat l'ordre
gdm3: opdracht niet gevonden
gdm3: parancs nem található
gdm3: perintah tidak ditemukan
gdm3: príkaz nenájdený
gdm3: příkaz nenalezen
gdm3: команда не найдена
gdm3: командата не е открита
gdm3: 命令找不到
gdm3: コマンドが見つかりません