How to apply effect into area

Software G'MIC
1

Right now, what I’d like to know is the most efficient approach to applying an effect into an area of a image. Can I use shared for that purpose? If not, then what should I do to apply effect into a section of the image?

+crop approach doesn’t seem to be a easy solution to this problem.

2

If the area is rectangular you can use shared. But assuming you have an opacity mask, probably the most useful is image (shortcut j) - it draws one image on another and you can supply a mask. Other than that there are a lot of blend commands.