Specular map creation with GIMP

[palistov]

Hey guys, haven't been on here for a while! I need to create a high-quality specular map for some textures. I use GIMP currently but cannot find a tutorial that walks me through the creation of a specular map with GIMP. I have found ones that give me greyscale spec maps but those don't seem to be what I'm looking for.

I've looked at the Amnesia files and there are many different types of specular maps they use. What sorts of methods might I use to manipulate the diffuse/normal map textures to create a decent specular map? I do understand that different materials require different methods, and it would be asking too much for someone to write out the procedure on this thread, but if anyone knows any links to beginner guides for GIMP let me know! Thanks!