gst-doc: 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/gst-doc - from package: gnu-smalltalk

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

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