Adzeth: There's also the whole "tell a company's lawyer that you're going to put their game on a torrent and they'll freak out and set your office on fire" thing.
timppu: P2P protocols certainly work better for DRM-games (like Blizzard's Battle.net games) where the downloaded games are useless by itself without an online authentication, but I guess it should be quite feasible for GOG to create a p2p protocol/client that tried its best to make sure you can't get bits of the files from other users you are not entitled to.
I can't tell if that would be technically feasible (and if so, how exploitable the result would be), but Adzeth does raise a very valid point. No matter how secure GOG could made a hypothetical torrent client, it still would mean that GOG needs to convince publishers to sell their games that way, which would probably double the publishers' resistance.
GOG: "Hi! We are a digital download shop, and we want to sell your games."
Publisher: "Ah, yes. We are very interested in digital downloads, but we are concerned about the ease of pirating such products. Which methods of protection have you planned?"
GOG: "None actually. We sell all our games DRM-free."
Publisher: "NO protection? Are you crazy? Illegal copies of our games would be all over the place!"
GOG: "It works, trust us. Ah, and by the way, we also plan to distribute your game on torrents."
Publisher: "Torrents? Isn't that the thing that the Prate Bay uses to distribute all those unauthorized copies of our games?"
GOG. "Yes. Why?"
Publisher: "..."