Not that kind of licensing problems. GOG needs not only the right to sell a specific game, they need the right to sell it for a specific operating system (depending on how the agreement is written). If the license holder originally agreed to selling the game for Windows (say, back before GOG added OS X support), GOG would not only have to get it running on OS X (or GNU/Linux in the future), but also negotiate the rights to sell the game for those platforms, regardless of whether or not it is the same (Windows) version running through Wine (or DOS version through DOSBox).
Ok, some right holders can be petty and selfish, but I never thought them to be total idiots.
If GOG manages to support their games on another OS, of course the rights holders would want them to: more sales, more money.
However, there are manpower costs for GOGs to get this done.
From an idealistic perspective, it is indeed strange that a website that claims to support DRM-free gaming would not support at least 1 free (as in free speech, but the free beer is nice too) OS.
In that respect, going to Mac OS before Linux is a bit of a slap in the face to some users as Mac OS is pretty much the anti-thesis of what free-er software is all about (Windows wants you to ask for permission before peeing, Mac OS wants that and also wants to go in the washroom with you to make sure you do it their way).
From a practical perspective however, what they did makes sense. Mac OS is approaching 10% of the home OS slice while Linux has like 1.5%.
Now, GOG probably has more than it's fair share of Linux users (the concerns about having DRM-free games and a free OS are similar after all), but I'd still be surprised if more than 10% of it's user base used Linux.
That being said, I think the distro concern is, to some extent, bogus.
Of course, nobody who is remotely sensible would expect them to support every 2 bit distro that is out there.
We are mostly interested in, maybe, the 5 main ones, so let's look at them: http://distrowatch.com
Now, out of the 5, you can take out Fedora, because it's a bleeding edge testing ground for Red Hat and nobody except it's maintainers should be expected to support anything on that distro.
Out of the 4 that remains, Mint is based on Ubuntu and Ubuntu is based on Debian. They have the same core packaging tools.
Supporting all of those 3 seems rather achievable.