gnome-session-save: 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-save - from package: gnome-session

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

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