Well... as Linux User I want play my games without reboot in Windows. So, I stopped using Steam for quite some time, but since they offer Linux "support", I use Steam every day.
In reality the Steam "support" sucks, (of course, this is in WIndows and Linux, I don't have Mac, but I'm sure too), yes, is painful, but me (and many people) continue shopping there. Why? Because the most times is my only option.

If GOG offers Linux version as "unsupported extras" or with the message: "This game is supported only for the developer", I will be happy and would not have to buy it on steam.

People in this thread have been claiming that GOG will inevitably support Linux and things like that. Well that obviously not the case. They continually insist on double standards for Linux. Not supporting a obscure Linux distro is no different than not supporting Windows 3.11.

Did you seriously feel the need to post this twice?

FYI, according to Canonical, Ubuntu 12.04 LTS will be supported for 5 years, and so will be the next LTS release in April 2014.

For future published games, if you asked the developer to build a .deb file and test it on the aforementioned distro before pushing the release to GOG, you wouldn't take too much of a risk, right?

Regarding your point on the amount of sales, I feel that DRM-free + Ubuntu builds would be a no brainer for lots of Linux gamers.

Recently many developers prefer a file .bin or a script (.sh) because this method works in many distros. Of course, they test only in Ubuntu, but with this method is easier for more distros (All games in steam are using this method).

This is the biggest contradiction at present. GOG customers seem to be more tolerant towards lack of support than GOG itself.

TET, why not provide the native Linux versions as unsupported extras? With a note along the lines of:

"Please note: GOG currently does not offer any Linux support. The developer of this game has produced a Linux version which we are making available for those that want it, however we cannot guarantee that it will work or provide any support; if you have any problems with it, please try the forum or asking the developer directly."

Seriously, why not do that? Sure it wouldn't be an ideal solution, but it would be better than nothing.

The way things are going I guess the Humble Store, Desura and Steam will be getting a lot more business from me as time goes on & more games get Linux releases, though I'd much rather buy them here :/

I don't have a dog in this fight--don't use Linux, or understand why anyone would want to use Linux--but after GOG decoded to include the Dark Fall 2 original version as an unsupported extra to Dark Fall 2: Director's Cit--and they don't even warn anyone that the original version is unsupported on the game page, I only know it's unsupported because a blue mentioned it on the forum somewhere--the whole "How could we possibly sell anything we don't intend to extensively support?" argument is kind of floundering. If that's seriously the big problem with supporting Linux, adamhm's solution seems perfectly fine to me.

I'm not sure if this idea was mentioned before (still reading through the massive thread), but what about a forum dedicated to Linux? Linux users could gather there to help out other Linux users. It wouldn't be official GoG support, but it would be some support from experienced Linux users. It would also gather all Linux questions/concerns in one area.

There are zounds of that :). Ubuntu forums, etc. Anyway, putting jokes aside, you can ask freely on the general forum or for specific questions on the game specific forums and we'll try to help if we can.

True. :)

So far, after searching, I've been able to find answers to my questions. I was thinking it would be nice if everything was gathered in one spot, rather than spread between general forums and game specific forums.

GOG adding subforums? Are you mad?!

Gosh, you'll be asking for an off topic section next.

I've never claimed sanity. :)

Supporting Linux releases shouldn't be a "huge financial cost". Maybe 14 years ago, but not today. There's nothing arcane about Linux anymore. My computer-impaired mom even uses it.

If doing so is a "huge financial cost" then somebody at your company needs to be fired, because it's that person's incompetence's doing, not Linux's.