GIMP Version: 2.8.16
Operating System: Windows
OS Version: Windows 7
GIMP Experience: New User

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I'm having trouble composing an image from layers. Steps I'm taking are as follows:
1. File->Open as Layers... (Select three images)
2. Image->Mode->Grayscale
3. Colors->Components->Compose
4. 'Compose Channels' Color model: RGB

Then, in the 'Channel Representations' I do not see any of the three layers created in Step 1 in the dropdown lists. The only thing in the dropdown list is 'Mask Value'.

How do I get the layers opened from separate images to appear in the dropdown lists?

I found out this appears to depend on how I create the images that are being imported. For some images, the steps above work fine, but for others it doesn't.
The real task I'm wanting to do is to take an image that is like the attached image 'snail_7.png' and recompose the three sections into a single image.
There's probably a better way to do this so suggestions for a better method would be welcome too.
I tried selecting the regions, cut, then paste as new image, and then export each image as PNG and the resulting images led to the non-working 'Compose' pop-up.
But when I create a new image the same size as the original and paste the cut section in, then export, the resulting images seem to work when following the steps above.
This has me confused. The attached images are the ones I exported and tried to re-open as layers. the ones with name 'good' work, but the ones named 'bad' don't.
Oh I can only attach three files, so there's one bad and one good.

My goal is to take the original image, made up of three sections of a grayscale snail images, and use the RGB compose command to recombine them into a single color image. With the top snail as the Red channel, the middle snail as the Green channel, and the bottom snail as the Blue channel.

A very similar question came up on another forum and you need to combine those images using layer masks.

I suspect that the images you have are not truly red, green and blue. I did try CMY as well not much success



(edit: I should point out that you can get this result with 'compose' but there is no way to adjust the colours from that dialog)

but as an example I did this little demo for the original question.
https://youtu.be/IKP2lm6ExQw 4 minutes, might point to a solution, you could play around with the layer colours & modes.