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1.
a route or way to an end, conclusion, or circumstance in which something additional or unexpected that is pleasing or valuable is found.

path to riches

travel with unexpected gain
after years of hard journey and sacrifice roadgravy was the ultimate prize.
by Darrell Ellzey August 07, 2007
 
2.
The shiny film that collects on your face during a long road trip in the sun.
"Man that was a long trip; I gotta shower off this road gravy before we hit the clubs."
by RBR August 07, 2007