gdmflexiserver: 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/gdmflexiserver - from package: gdm

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

On Debian possible default full paths are:

/usr/bin/gdmflexiserver - from package: kdm-gdmcompat
/usr/bin/gdmflexiserver - from package: libgdm1

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

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