dhcpcd-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/dhcpcd-gtk - from package: dhcpcd-gtk

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

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