When i use the GOG downloader i click the magnifying glass next to the download once i'm done.

I then use the setup file, and download my game. My game is then downloaded to another folder, whilst the setup files remain. However, the setup files take up a lot of space. Should i delete these, or do i need them for the game to function ?

Nope. You can delete them. You only needed them for the install.

Could you say exactly which files are needed and which can be deleted, and in which folders these files are?

The deletable files are named something along the lines of "setup_planescape_torment". Their location depends on your settings; either they are in your "Downloaded Files" directory (if you downloaded them using just the browser) or "My Files/GOG.com downloads/planescape_torment" (if you used the GOG Downloader). They can be somewhere else, too, if you've got your browser or the Downloader configured differently.

Where the game was installed is anybody's guess (settings again), but I think it goes to "Program Files/GOG.com/Planescape Torment" by default.

I use the GOG downloader and have a folder named GOG.com Downloads, which is accessed when clicking the magnifying glass from the downloader's finished downloads. All of the files in this folder along with it's sub folders start with setup or patch. The biggest of these folders is The Witcher with setup files adding up to over 8 gigabytes. Are these the files that are safe to delete?

I also have to other folders named GOG.com. One is located in Program Files, while the other is located in my Start menu.

The setup files are just that. Setup files. Think of them as the dvd you use to install your game, that is then no longer needed in the drive. You can safely delete them, though if you want to reinstall the game you will need the setup files again. Depending on your connection (and installation frequency) you may want to keep a local backup.
The games do not need them to function though.

Yes. Nothing in the "GOG.com Downloads" folder is necessary to make the games run after they have been installed and patched.

E: Ninja attack!

I just got a 20 mb internet, so I think it's safe to delete em.

Thanks for the help everyone ^-^

I'd recommend storing them all in some extra folder, maybe on an external HDD. That way you have them readily available, and don't need to download them again. With high-speed internet that's probably no big deal, but aside from your own bandwidth, you also help reduce unnecessary server load for GOG, which in turn reduces costs and also could help improve download speeds for others. Also, in the unlikely scenario that the GOG website ever goes kaputt, temporarily or permanently, you still have your own personal back-ups of your purchased games.

So, yeah, I'd recommend you keep your own back-ups of your installers, but it's not a requirement -- It's up to you. ( By the way, if you downloaded any bonus extras with your games, those should be in the same folders as your installers. )