Some games on GOG have full nudity in them, yet you can buy them as a minor, is this not illegal?

nudity =/= pornography

According to US law, no. Ratings of videogames (and movies for that matter) are not done by any government group, and any policy decisions surrounding the sale of games with various ratings to minors are left to individual retailers with no laws regulating what policy they must set (ESRB ratings and individual retailer policies have basically been to prevent any government intervention by dealing with the matter in-house). The Child Online Protection Act (passed in 1998) may have made the online sale of games containing nudity to minors illegal (depending on its interpretation), but that law was struck down by the Supreme Court in 2004 (after working its way through the lower courts for several years).
There may be some countries out there where it could be illegal, but as far as the matter applies to your purchase of games through GOG you can buy any of the games containing nudity and it is perfectly legal.

It is when you are in an ingame porn store.

I bought Mass Effect the other day (rating: 18 and over) and all I had to do is tick a box to certify I'm over that age. That's all that seems to be legally required.
Moral:
Anyone of any age with a credit card can buy anything on the internet these days. Companies have no way of knowing who's on the end of a keyboard, and have no way of knowing unless a system is developed that checks individual's data records (e.g. via bank or passport data) - God forbid if that ever happens.
GOG et al can only take it on good faith that people tell the truth when they purchase, and that parents are diligent and monitor what their children are purchasing.

Since GOG is based on Poland, I think Poland's rating law (if one exists) is the one applicable here. Any Polish wants to give us insights on how game rating works down there?

In Poland, it's legal to drink at age 6 months! However, it's illegal to buy pornography unless you're 75 or over.

Typical. You get to the age you can enjoy pornography and you cant even get it up.

I can't even think of a single occurrence of age ratings being actively enforced.
So surely no need to worry?
I think the assumption on the internets is that if you have a credit card, then you must be old enough to access whatever media you have purchased.
Which games on here have any level of pornography or nudity? Duke3D? Seems exceedingly tame to me, but then we do have page 3 girls in my country... (topless girls on page 3 of a couple of "news"papers.)

Beneath a Steel Sky also has some obvious T&A in it, but I would hardly classify it as porn.

And yet, that's exactly what Labour are pushing with their "identity card" screw up...only, their databases aren't going to be secure in any reasonable fashion, they can be cloned with very little hassle using cell-phones widely available today (even though they deny this, it's been repeatedly proven beyond a doubt), and ANYONE off the street can access any little piece of information on you without your knowledge, or consent....

Just read this:
"The Polish sex industry held its first legal trade fair in September 1998, attracting businessmen, pornographic film stars, and both male and female participants. Spectators at the Warsaw show will be treated to erotic dancing, mock sado-masochistic performances and naked beauty contests for both sexes. They can also browse around stalls offering sex aids, films and books. ("New law allows Polish porn festival," Reuters, 25 September 1998)"
I wanna go to Poland.

Dude, I haven't seen any NUDITY at all (Except with the XXX pack on Duke Nukem...I watched a trailer, for shame.). I can't buy any of the Postal games nor Waxworks, and I'm flipping 15 years old. Yet I can buy the others ones....Sides, Duke Nukem (Mentioned in your other comment on another post) was made in 96....things aren't the same.

Postal and Waxworks are up for grabs to anyone.

Says you must be 18 or old to purchase for me.