I'll grant that Desura is definitely a small player.
I've thought of GG as being a minor player but legitimately mainstream and big enough to worry, if not threaten, Steam.
OTOH I've always thought of GOG as owning a niche (legacy titles) that nobody else is much interested in, its very name sort of denotes how it's restricted.
I've always thought of GOG as owning a niche (legacy titles) that nobody else is much interested in
They started that way to gain the traction, but they definitely don't plan to stay that way - they add new titles and aren't focusing on old games exclusively anymore. I view them as a solid competitor to Steam which will threaten them even more in the future, since they have a resolute DRM free stance. Linux additions would boost GOG position a lot IMHO. They still work on adding old titles though, since that's their original specialty and that's a good thing.
I'm not sure about Gamersgate - never used them. Are they DRM free? If not, they aren't really offering anything better than Steam does, and won't compete with GOG here.
That's surely better than Steam already, which doesn't help filtering DRM free titles. Though Steam in general imposes its client DRM, even if the game itself doesn't use some heavier forms (like on-line type).
