gnome-system-monitor: 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-system-monitor - from package: gnome-system-monitor

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

On Debian possible default full paths are:

/usr/bin/gnome-system-monitor - from package: gnome-system-monitor

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

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