So going by this article it seem rating a game can be quite expensive. Maybe this xan explain why some games aren't on GOG?:

http://www.polygon.com/2014/8/1/5959285/xbox-one-indie-game-cost

Those rating boards remind me of this song:
https://www.youtube.com/watch?v=wTP2RUD_cL0
:)

I don't believe that is the reason. Defender's Quest is unrated, e.g.

Maybe not for Defender's Quest but for some games maybe it could be. I don't know what the legal requirements are in different countries but that is not all that matters it is also important what various companies think those legal requirements are and what risks they are willing to take. That would certainly also depend on the content of the game I would think.

Rating games is very expensive indeed, but that wouldn't explain why games aren't on GOG, because GOG doesn't require game ratings. If they're available, GOG will sometimes publish them, but they need to exercise caution. A German USK rating, for example, may potentially only apply to the German-language version and not cover the English version.

It's not like Nintendo, Sony, Microsoft or EA, which requires games to be issued with an age rating certificate of the country in question before the game can be published. GOG and Steam don't enforce age rating policies.

It does partly explain, however, why Shovel Knight has been delayed on the EU Nintendo eShop.

Well you mention EA as a company that requires rating. That could impact GOG. If EA has a corporate policy of all their games being rated and some older game they might otherwise release on GOG is only rating in a few territories they may consider it too expensive/risky to sign global rights to GOG. Other companies may have similar concerns. GOG not requiring ratings doesn't help much if the publisher, developer or some government entity requires ratings. Notice that I offer this only as a potential explanation in some cases and that article is interesting independently of the impact ratings may or may not have on GOG.

For example we have here an obvious explanation of the annoying practice of regional restrictions. Which leads me to a question. Does anyone here know if the GOG site is accesible everywhere? What about countries like Iran? Anyone have any knowledge about that or experience trying to access the GOG site from a country known to censor the internet?

I don't think you understand. EA doesn't require ratings for all of its games. If it did, it'd be pretty fucked in terms of its Android, browser and iOS offerings. EA requires age ratings for stuff published on Origin, which includes non-EA games.

GOG doesn't require age ratings, although that does expose them to certain legal risks in countries like Germany. There are many problems other than censorship. Specifically, GOG subjugates itself to Californian law, which itself causes problems when dealing with Iran. I think that GOG is accessible from Iran, but it's a moot point, because nobody can pay for anything, as payments go through American providers.

It's a safe bet that GOG is not accessible from North Korea.

Edit: Voila. Proof

It does? From the Terms of Use: So if it is a service targeted at adults, does the rating matter? One could say that they are not properly checking their users' age, but not sure if the rating would matter.
Hm, time to go poke Simon it seems.

That doesn't satisfy German law, which requires websites to enforce effective age check measures for games rated 18 (unrated games are a bit of a sticky point, as most indie games are unrated). There's also a prohibition on the open, unrestricted sale of blacklisted titles, some of which are here (Blood, Cannon Fodder and a few others).

Of course, in the case of GOG, it can be argued that the site isn't even targeted at German users, as its transactions are largely denominated in dollars and the site is in English, so it's unlikely that the government will take aim.

For what it's worth, Steam also does a pretty sloppy job of enforcing German law. On the one hand, they seem to take a pretty heavy-handed approach regarding the sale of blacklisted games, but at the same time, they completely ignore the requirement of effective technical measures surrounding 18-rated games (although, as I understand it, the implementation of the family options does now satisfy the requirements, as useless as they are).

The only one that does it properly is Origin, which imposes a purchase time window for 18-rated games of 23:00 until 06:00, which essentially makes it unusable.

(Just out of curiosity, what happens if you set your DOB in GOG to be under 18?)

You apparently can't buy Phantasmagoria? I seem to recall an underage GOGer complaining that he couldn't purchase the game.

As Grargar said, you are unable to buy some games. I think it was Rohan15 who reported this, not sure what games that was for though. I can try to dig up the post if you want though.

I didn't think that porting games/releasing games on Xbox One was going to be cheap but that expensive is a little crazy. You'd think that if Microsoft wanted to get new developers to create games for the console that they would help them out a bit, maybe offer Microsoft E&O insurance for a low rate.

I do know that it costs money to patch games, well at least on last gen it did. From what I heard the first patch was free for the game, than subsequent patch's cost money.