gtk-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/gtk-demo - from package: gtk2-devel

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

On Debian possible default full paths are:

/usr/bin/gtk-demo - from package: gtk2.0-examples

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

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