Masks - Can't seem to get it right

[Ofnuts]

Yes, GMIC is a bit fiddly. You can tell it to use All layers, but first and foremost, you need to have two identical image that only differ by the colors. And the images you show haven't even got the sale size. So your first task is to use the Unified Transform tool to make them overlap, and then crop the image to some common part. Then you can use GMIC, asking it to create a new layer, and it will create this:

   

As a test, reapply the CLUT to the first image, and you get this: