For one thing, x itself is the displacement from the location, x=0.Thank you. I did follow these questions which had led me to that result.
x+h? as h being the displacement from x=0?
(If you are taking GPE to be zero at x=0, then h should not enter into this anymore.)
I asked for the GPE (gravitational potential energy) of the mass when it's at location x .
This should help you answer @haruspex's question(s)