You don't need any channel.
- create a selection
- make sure the source layer is active in the source image
- Edit>Copy or Ctrl-C: your selection is copied in the clipboard
- move to the target image (if copying between images), make sure the target layer for the copy is the active layer
- Edit>Paste or Ctrl-V
- You get a "floating selection" layer that you can adjust (move around, etc...)
- Layer>Anchor (cCtrl-H) to merge down to target layer, or Layer>To new layer (Ctrl-Shift-N)