when i download a game through the launcher "gog galaxy," i can't do a backup copy. I tried to copy the main directory folder of The Witcher 2 on my hard-disk, and when I try to start it on another PC, the game does not start. While downloading the game directly from the site it works...

You can download the installer via Galaxy, too. Just take a look at More ->Backup and Goodies

You can download and run all your games without Galaxy. Just go to your library on gog.com, download the installer and have your DRM-free copy of every game you own on GOG.

Once a game is installed, it's bound to this computer (especially with Windows). To run it on a different machine simply run the installer there again. The only thing you'll have to take care is to manually transfer your savegames, since GOG doesn't support automatic cloud saving (yet).

Quite a few games may require additional items other than the game folder, including registry entries, frameworks and/or items in other folders (like the documents one). You can either import the game through Galaxy on the other computer and let it repair/install what's missing, or you can use the standalone installers that you can get either through Galaxy or through the browser. I have moved TW2 on other computers, and after having Galaxy import the folder, it worked just fine.

DRM-Free does not mean portable.

DRM-Free doesn't mean 'portable'.
You still need the installer to create the registry entries and such and the installer can be downloaded via the website or Galaxy.

There's a huge difference between an installation file and an already installed game. You can copy an installer and it will work as many times as you want. But copying a folder with a game isn't the same as installing a game. During installation things like registry entries are added, separate directories for saves and profiles, additional libraries/runtimes etc.

E: and there already is like half a dozen similar answers... :P

BLASPHEMY!

More seriously, I'd go to Account->Library, find the game you are interested in (in this case TW2) and just download it from there. Although I'd prefer use of the GoG downloader, you can use the web browser and download it in individual parts or as hohiro has said, Galaxy offers an option to download the installer.

Well, this is a new one. :P

I would be awesome if the games had portable versions here on GOG. ^_^

Some might, but you'd have to experiment to find out which. :)
(I'm going to avoid asking why that would make you awesome... :P )

I'm going to wager any games using DosBox are 'portable'.

I do enjoy games I can just unzip and play, and then delete. No mess, no fuss, no registry management, tinkering or complex configuration.

Yeah, for games and certain other software that's nice (though I can understand why it's not always possible). For some types of "utility" software (such as VLC), I actually prefer to install it though, rather than simply "unzip & plop".

Tools and utilities, sure. But games?

Most (if not all) software should be portable and just ready to use, unless there's a good reason for it not to be. And most of them I don't see a compelling enough reason.