YellowAries: I don't think so... I could be wrong, but I highly doubt it..
I'm pretty sure in the vast majority of cases it's material owned and provided by a game's rightsholder (box art, concept arts, old marketing material etc), then only edited, rearranged etc. by someone at GOG. There may be cases where GOG has to create material of its own and/or use stock images, maybe they sometimes even seek out someone who has created nice fan art a while ago to get a license for using it, but I'm pretty sure that's exceptions.
jcoa: If gog made their own box art for a game, could the publisher use that art outside of gog?
Unlikely but that's impossible to tell for sure since we have no direct insight into the legal stuff happening between GOG and the publishers/developers (at least not that I know of). My guess is, though, that if GOG actually has to obtain artworks from another party than the game's rightsholder or to create an artwork from scratch only they can use it unless another arrangement is made.