pygtk-demo: 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/pygtk-demo - from package: pygtk2

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

On Debian possible default full paths are:

/usr/bin/pygtk-demo - from package: python-gtk2-doc

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

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