DanTheKraut: Again for Germany this is wrong...
this is the legal situation for Germany:
The games aren't banned in Germany at all. Some are indexed like Quake 2 or 3 and Spear of Destiny which means adults only (commercials and public selling only in places adults have access too any other store can sell those games too but you need to ask for them).
In fact there are 3 rated 18 ratings for games in Germany.
USK: KJ and unrated (which automatically means rated 18) - those games can be sold anywhere and also commercials aren't a problem.
Indexed - Commercials and public selling are restricted to places minors don't have access but any store can sell those games with the only exception that you have to ask for it.
Then there is also a difference between download titles and retail versions when it comes to laws.
Wolfenstein 3 and RTCW got seized by a court which means no commercials at all BUT import/buying and owning are still legal
if you are an adult. Selling is a bit complicated in this matter but also not forbidden (Several court decisions regarding this also from the highest court here).
GOG or Steam are not affected by this because the JMStV (for download titles) and the JuSchG (for retail versions) only count inside Germany and for stores placed in Germany.
(snip)
And because you will not believe me a official source for Germany
Can I distribute an unrated or “indexed” title in Germany?
Yes, distributing games without a USK rating in Germany is in line with the existing statutory Regulations. Certain precautions have to be considered though: if your program clearly is a game (and no “infotainment” or “edutainment” program) then your title will be labeled as “unrated”, independent from the possible age classifications of other countries such as PEGI, ESRB, BBFC, CERO etc., and
can therefore only to be sold to adults. If an allegedly gameplay-identical version received a USK rating at some point is negligible for this matter. Titles without USK rating are also endangered to be placed on the prohibited list (“Index”) by the Department for Media Harmful to Young Persons (BPjM).
If the respective title has been indexed by the BPjM, specific terms of sale apply These media must not be displayed, offered, announced or advertised publicly in places accessible for minors. While this general ban of advertising exists for these titles (in accordance with Article 6, Section 1, Clause 6 JMStV),
the sale is also restricted to verifiably adult persons only. http://www.usk.de/en/extramenue/login/publisher/start/publisher-faqs/
DanTheKraut: (snip)
GOG is a store with a German website and targeting customers in Germany. As an online store it therefore has to abide by German laws.
Which means that the bolded parts about selling to adults do pose a problem for GOG.
This article explains the hurdles to sell those games to adults in Germany as an online distributor pretty well and makes it clear why GOG and Steam don't do it: