gnome-session-selector: 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/gnome-session-selector - from package: gnome-session-custom-session

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

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