When I think of what GOG can do to support Linux, porting their games is not one of them so we will ignore that.
I would argue that at least 40% of their catalog are dos games run in a custom dosbox (I could be wrong about that). If that is the case, then why not use dosbox on linux? You already own several games that have Linux (and/or Mac) versions.
Too many variables on Linux (i.e. Distributions, drivers, desktop environments)?
Then just use Ubuntu 12.04 LTS. It is a Long Term release which means it will be supported up to 5 years, just a year after the next LTS release. That would mean you only have one distro to work with. All of the spinoffs would work as well (like Linux Mint) and that combined will cover about 40% of all home Linux users. If that seem like to much work on your own, Canonical would be more than happy to help you in their efforts like they are doing with Valve/Steam.
Not big enough market for Linux?
Next year it is projected that 5% of all computers in the world will ship Ubuntu. Combine that with the million or so current Ubuntu users PLUS all the Ubuntu spinoffs and you will see that there is quite a big number there. Many of those users are hungry for quality gaming. Linux might be only 1% of the entire computing world, but I’m guessing less that 30% of all Windows users play games (excluding web browser/flash games). While the same can be said about Linux, as more games (and other software) become available, there will be an increase in the number of Linux users thus an increase in Linux gamers.
On the negative side of, while the Humble Indie Bundles have shown that Linux users are willing to pay twice as much as Windows users, Linux and Mac sales combined equal only to half the Windows sales BUT that’s still a lot of money.
P.S. While I am a Linux user, I also stand by Mac brethren. No one should be withheld from good games.