gdm-control: 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/gdm-control - from package: openbox-gnome

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

On Debian possible default full paths are:

/usr/bin/gdm-control - from package: openbox

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

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