Does anybody know if you can download a game off GOG or where to find the source folder of the game? I'd like to play some of my games from GOG on my old laptop which has no internet. So I need to figure out a way to transfer the games from their folders to an external hard drive or USB to transfer them to my laptop.

:)

Just download offline installers and do whatever you want:
1. use web browser: https://www.gog.com/account
2. use GOG Galaxy to download offline installers
3. use the unsupported old official GOG Downloader to download offline installers
4. use 3rd party tools to download everything you own at GOG:
4a: gogrepo.py
4b: LGOGDownloader

1. For single GOG games:

Download the offline installer of the game from your GOG account. You can do this e.g. with your web browser, or using Galaxy (look for how to download "backups" with Galaxy).

https://www.gog.com/account

If you are using your web browser to download a game installer, it is advisable to download the "classic" version which doesn't have a Galaxy installer embedded into the game installer, as you probably don't want to accidentally install the Galaxy client along with the game on that internet-less PC. The embedded Galaxy client increases the game installer size by around 120 MB (for no benefit to you).

Look under "More => Classic Installers" to get the Galaxy-free version of the installer. Not all games have Galaxy-embedded version anyway.

If you use Galaxy to download the backup installers, they are the Classic installers by default.

If you downloaded the Galaxy-embedded installer anyway, just make sure to disable Galaxy installation from the game installer options, before you start the actual game installation.

2. For downloading all your GOG games (as offline installers):

Learn to use a third-party tool like gogrepo.py, or if you are a Linux user, lgogdownloader. They are mass-downloading tools created for easily downloading ALL your GOG game installers in one swoop.

https://www.gog.com/forum/general/gogrepopy_python_script_for_regularly_backing_up_your_purchased_gog_collection_for_full_offline_e/page1

https://www.gog.com/forum/general/lgogdownloader_gogdownloader_for_linux/page1

EDIT: ninja'ed

Not sure how new you are, so some of this you may already know....

Once you buy a game here you can go to your games, a list of all the games you own here.

Click on the game you want to download, you will see a list of files (for small games maybe just a file).
Larger games have an exe file and one or more bin files. All should be downloaded to the same location, usually "downloads" in Windows.

Click on a file you will asked for a download location (usual default is downloads).

Once all the files are downloaded, run the exe and the game will install. The downloaded files remain where they were. This is the "installer". This is what you can move, copy etc wherever you want.

As far as I know, the largest games are split into Exe and several bin files, none larger that 4GB. So a 4+ GB usb stick will allow you to transfer any installer to different pc. Once the Install files are transferred, run the exe and it will be installed in the new pc.

DO NOT abuse this. It is the non-DRM that GoG is famous for.

In other news....
There is a thing called galaxy, I know nothing about this other than it's "meh" reputation.
There is also GoGdownloader, maybe still working, not supported.

I hope this helps, and that you enjoy an abuse free experience.

Post back here for further clarification.

Edit, double ninja'd. My, what a helpful bunch we can be.

I just drag the installer file to my external hard drive after downloading them.

I use the GoG Downloader linked to a few posts back.

Thank you OP for asking this question and thank you everyone for the very Detailed information!! I also was looking for much of the info provided. :)