Hello there. I noticed, that gog games are wrapped inside at least gog installer, and dos games are wrapped inside dosbox (well, I assumed it only by testing one, free game - Teenagent ;). So are they designed to run only on xp & vista, or for example Commandos 1 will still run on P100 & 32MB ram? Is there any overhead because of gog's additions? And I would also like to know, whether games with 3dfx Glide support still support it and run on, let's say, win98? Thanks in advance for any infos.

The games are meant to be played on newer hardware and operating systems.
If you want to play on your old 486, you'll just have to manually transfer the files after installing and apply some skilled hacking and you might be able (or not) to make it work on a game-by-game basis.

For the most part you can check the system requirements. Seeing as many use emulation they require a more powerful system. But no, the games aren't designed to work on older systems.
The bottom line is you'd be better off looking to eBay.

What about Glide?

Well it hasn't been around for about a decade now so there's no reason for GOG to support it. A Glide option may still exist in the game. But it probably hasn't been tested. Some of the code that has been rewritten may render it incompatible with Glide if not the pre-XP OS you're using.

The whole point of GOG is to be able to run old(er) games on new(er) hardware. If you have an old machine, then look around for the original games. Some of the games GOG sells like Beyond Good & Evil or Prince Of Persia wouldn't run well in a P3 or older machine anyway.

This is correct.