There is an Urban Dictionary entry saying
Of Old English origin, shortening of "Good Speed," and contrary to popular belief has nothing to do with God. People would say this to others who were leaving on a long and perilous journey, wishing them success, but more importantly wishing for their return.
Edited: Can it be used without invoking any nuance of religiosity ?
Well guys, I did check the references and did find the explanations which ended up becoming the answer for the question. What I meant was to get an educated opinion on the usage of the term in modern times.
How a question with so many votes gets put on hold is beyond me.