gtk3-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 Debian possible default full paths are:

/usr/bin/gtk3-demo - from package: gtk-3-examples

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

Error in other languages:
gtk3-demo: bevel nie gevind nie
gtk3-demo: comandă negăsită
gtk3-demo: comando não encontrado
gtk3-demo: commande introuvable
gtk3-demo: command not found
gtk3-demo: käsku ei ole
gtk3-demo: không tìm thấy lệnh
gtk3-demo: komanda nerasta
gtk3-demo: Kommando nicht gefunden.
gtk3-demo: kommandot finns inte
gtk3-demo: komut yok
gtk3-demo: nie znaleziono polecenia
gtk3-demo: níor aimsíodh an t-ordú
gtk3-demo: no se encontró la orden
gtk3-demo: no s'ha trobat l'ordre
gtk3-demo: opdracht niet gevonden
gtk3-demo: parancs nem található
gtk3-demo: perintah tidak ditemukan
gtk3-demo: príkaz nenájdený
gtk3-demo: příkaz nenalezen
gtk3-demo: команда не найдена
gtk3-demo: командата не е открита
gtk3-demo: 命令找不到
gtk3-demo: コマンドが見つかりません