Line to GRN or Neutral to GRN blowing my GFCI?
It could be either one.  The point is the loop is leaking to earth, and it shouldn't do that,  and you should go through it and look at all the connection points.   

It's unlikely to be the cable unless he used indoor rated cable.  

DVMs are inconclusive.  Obviously a low voltage reading is a problem, but some ground faults act like VBOs - they don't actually suffer insulation breakdown until a certain voltage is reached.  Or they may require a certain operating temperature, etc.

Your starting point is to give all wire endpoints and connections a good once-over. Do that first, because you'd feel pretty foolish doing an exhaustive technical troubleshoot only to trace it to the lamp and find a mouse nest at the terminals lol...