Siehe Wörterbuch: godspeed

It's one word, with no apostrophe-S, and (despite the LEO entry) usually uncapitalized. It just means a safe journey, a good trip, or sometimes just good luck in a general sense. It's now rather old-fashioned.

It comes from 'May God speed you,' in an old sense of 'speed' that meant something like 'cause you to prosper,' 'give you success.'

Perfect and exhaustive explanation, thank you, hm. Before I checked the Forum, I had already found "God speed you" in Leo. I was not sure, however, if god as a noun existed. I guess the translation could be then "Gott sei mit Dir".