Ok, your in big trouble here. The problem is that because they are photocopied then its turned all the lines into uniform color so there is no color difference between what you want and what you don't want. Next is luminosity - same. Its either black or white. That leaves the pattern and the pencil lines are about as thick as the grid which is also difficult to distinguish between them. So your not left with many options. I have a very special program which is not part of Gimp native but can be accessed with some extra plugins or something - I dont do Gimp. Anyway, this program looks for a grid like pattern and then removes it but in that process will remove some of the line info as well. You have the almost pathalogical case to deal with.

Can you get the original non photocopied versions of the maps and get them scanned in with a flatbed in color or at least greyscale so there is something to key on. Here is my best shot. Its not very good I would admit.