Do you know the meaning of 'Godspeed'?
According to wikipedia, Godspeed is a wish for a prosperous journey, success, and good fortune (from Middle English God speed you , meaning "May God help you prosper").
Godspeed is heard often during the Yultide Season (Christmastime) when people are traveling and visiting their loved ones. It is short for May God speed you along in your journey, probably.
What's the difference between 'Good luck' and 'Godspeed'?
Good luck is a general wish that everything turns out ok in whatever you are undertaking.
So, good luck in your english studies, and Godspeed [on your journey learning English].
Good luck!:Boa sorte! - Usado em qualquer situação onde se queira desejar boa sorte a alguém.
Godspeed!:Que Deus o acompanhe,boa sorte na sua jornada! - Já é uma expressão considerada antiga.