Using GIMP I would like to create a color-separated PDF. I plan on making printing plates for a home letterpress and they request a color-separated PDF, which seems to just be a page for each color but I don't see a way to do this in GIMP.
One way to separate the colors is to merely turn off all but one channel and then export a pdf of that channel as a grayscale. What software are you using to create the plates and why PDF?
Since there is a CMYK issue with gimp, you really only have R G and B plates.
You're sort of in a tough spot if you want to use GIMP for print, since it doesn't support CMYK colorspace. You can try the Separate+ plugin, but it does have some limitations (though they may not be an issue for what you're trying to do?)
More in-depth reading about GIMP & CMYK issues: https://wiki.archlinux.org/index.php/CMYK_support_in_The_GIMP