They appear to all be gone. I've been using them and in fact was just about to redownload my Fallout installers (they failed the check), but now the checksum topics are gone.
Have they been moved or something? They're very useful.

Yes, the installer now does a check.
You can still find them over at this location
http://www.gog.com/en/support/gothic/MD5_checksums_for_older_installers_
Ignore the 'gothic' part in the URL (but leave it there to see the page!), you get the same info on any game. And access this from your "My Games" support page.

Ah, ok. Now, I'm no expert, but I am confused. Obviously I don't understand enough about file integrity checks, but I thought the point of having the md5 reference for for reliability. How could, for example, an already corrupt self-checker be trusted to check itself?
Please humor me. This is curiosity; I already consider the original question answered.

While the installer may of downloaded enough to perform the check, it does not mean the entire file is valid against the md5.
Many-a-time I've downloaded large files, that 'work' to a certain point due to a bad/incomplete download.

Think of it like a box of cereal. :3
Let's say the cereal box says exactly how many tiny pieces of cereal should be inside.
If you can't read the box, it's probably broken, and you'll already know something's wrong!
But, as long as you can read the box, you'll know exactly how many pieces of cereal you should have, and you can compare and check its contents.
Now, let's call the box the 'install program' and the cereal the 'game data'.
As long as you can read the 'installer', you'll know exactly how many pieces of 'game data' it should contain, and can compare and check its contents. If you can't read the 'installer', it's probably broken, and you'll already know that something is wrong!

So self checkers never give false negatives, you're saying. The box can't be broken in such a way that it appears to be readable but is giving the wrong sum?

EDIT: It's possible that the installer would give a 'wrong sum' if damaged (incredibly incredibly incredibly unlikely), but...
Even if the box/installer was only slightly damaged and gave a different sum, it's pretty much impossible that the damaged sum will match the contents.
The 'wrong' sum would say that game is damaged, which it is, because the installer (which includes the game data) is damaged to give the wrong sum. It's pretty much fail-proof.