Here is a generic way to proceed, from a mask :
gmic image.jpg 100%,100% -circle[-1] 50%,50%,120,1,1 --blur[0] 10 -j[0] [-1],0,0,0,0,1,[-2] -rm[-2,-1]
First you load the image to filter, then you create the mask, then compute the result of the filter, and finally you draw the result on the original image according to the masked region.