gst123: 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/gst123 - from package: gst123

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

Error in other languages:
gst123: bevel nie gevind nie
gst123: comandă negăsită
gst123: comando não encontrado
gst123: commande introuvable
gst123: command not found
gst123: käsku ei ole
gst123: không tìm thấy lệnh
gst123: komanda nerasta
gst123: Kommando nicht gefunden.
gst123: kommandot finns inte
gst123: komut yok
gst123: nie znaleziono polecenia
gst123: níor aimsíodh an t-ordú
gst123: no se encontró la orden
gst123: no s'ha trobat l'ordre
gst123: opdracht niet gevonden
gst123: parancs nem található
gst123: perintah tidak ditemukan
gst123: príkaz nenájdený
gst123: příkaz nenalezen
gst123: команда не найдена
gst123: командата не е открита
gst123: 命令找不到
gst123: コマンドが見つかりません