gnunet-fs-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 Debian possible default full paths are:

/usr/bin/gnunet-fs-gtk - from package: gnunet-gtk

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

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