You have to ask yourself a lot of questions as to what you are striving to do with these 2 different breeds. The black copper marans are known to have the darkest eggs of that breed and the GCMs offer color but not so dark eggs. Body size of both birds are similar but I would say the marans are a tastier bird. They both lay about the same number of eggs yearly so there is really no gain in that respect. Just what is it you are striving for? Personally, I dont see many or any gains.
If you have got caught up in the eye candy loop where you enjoy color of a nice looking hen, might I suggest the bielefelders and the neiderrhieners. If you enjoy the orpington breed, look into a lemon cuckoo orp and breed that with a marans. Just some input and nothing more. Enjoy and have fun!