Copy channel from one image to another image.

[Ofnuts]

Make the source image active, open the Channels list, and drag the channel to the canvas of the target image.
This creates a layer in the target, so open the Channels list, and drag any of the R/G/B channels down to the Channels list to copy it. Once done you can remove the layer.

If you want to copy the selection; set both images to quickmask mode, Edit>Copy in source and Edit>Paste in target.