One way:
- Open your other image
- File>Open as layers and open your Scotland image
- Move/Scale Scotland to match
- Colors>Invert to make it a white on black image
- Hide your other image layer
- Open the Channels list and drag one of the RGB channels to the general list to copy it (that should be your Scotland image). Make sure that all chanles remaine selected (dragging the channel xwill have deselected it)
- Remove/hide the Scotland layer
- Select the other layer and make it visible again
- Layer>Transparency>Add alpha channel (if greyed it is already done)
- Layer>Mask>Add layer mask and initialize to the copied channel