Hi Liu,
Sorry for the trouble here - you are correct, right now there is no way to create a reference to this result variable. We'll look into adding it in an upcoming version of LEAP.
In the meantime, using the Results view you could generate the indicator you describe, by selecting the result Demand:Primary Requirements: Allocated to Demands, and on the y-axis, select "Denominator" and navigate to the appropriate GDP assumption in your model. You can then save the chart as a favorite to quickly return to it, if you wish.
Best,
Taylor