gforth-fast: 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/gforth-fast - from package: gforth

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

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