[PD] GEM blurring

[jma at jeanmarie-adrien.net]

Hello list
I need to blur a little portion of an animated image with GEM to achieve dynamically a good visual continuity between foreground and background : 
means blurring the edge between foreground and background. 
This edge is not moving in the frame, just the content of the background varies. 
What would be the object(s) for doing this ?
tks a lot
JM