Select a region, choose Edit > Cut Image or Edit > Copy Image
, then Edit > Paste Image
.
See Create a mask by selecting a region.
You can also select a region of an image layer by creating a mask for that image layer (see Create a mask from an image file, Create a mask by painting, and Create a mask from curves).
Choose Edit > Cut Image.
Choose Edit > Copy Image.
The cut/copied region is pasted into a new image layer above the active image layer.
The region is pasted either into the center of the selection mask (if there is one) or into the center of the canvas plane (if there is no selection mask).
In the application, choose Edit > Paste.