Do the programs you re using allow an image to be in greyscale mode? This doesn't mean that the image is colored in all grey; no, that is RGB color.
First, if I remember correctly, you've downloaded Gimp off of an illegitimate site, yes? Bad downloads that are not official usually are laced with other malicious programs and removed aspects from the original, official download. As long as you download GIMP from their official site, your laptop should remain perfectly fine, file-wise. I searched if FireAlpaca had a greyscale option, and alas, I had found no direct responses, so I am unsure if it does support greyscale images. I do not use this program myself, and if it does have this option, someone correct me!
If those programs you have used to not have a greyscale option, you will have to download a program that does, or have someone re-make the heightmap for you. I will be willing to make/remake you a heightmap if you PM me all of the information. If you choose to not download a program that does support greyscale images, there's only so much we can do. Good luck!
As for the game unable to find the file, make sure it was placed in Feralheart\media\terrains, and make sure you include the file extension when typing it into the game.