bpython-gtk: 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/bpython-gtk - from package: bpython

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

On Debian possible default full paths are:

/usr/bin/bpython-gtk - from package: bpython-gtk

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

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