Luminence drives effects values

Assuming your gradient map is a comp-sized layer centered in the comp, and you want to sample an area of the gradient under the anchor point of the shape, you could use something like this for the Choke Matte property:

r = .5; // sample radius
maxChoke = 50; // max value for Choke Matte

L = thisComp.layer("Gradient Map");
p = transform.position;
c = L.sampleImage(p,[r,r],true,time);
hsl = rgbToHsl(c);
linear(hsl[2],0,1,0,maxChoke)