So i bought a 15tb hard drive and i dont know which would be the best way to back up my games.Should i put gog galaxy on the hard drive and download every game with it or should i go to the official site and download the offline installs?I think both ways are fine tho....

I would download the offline installers, no need for Galaxy, but at the end of the day it's your choice. By the way, how much did it cost, the HDD I mean?

For backup always download the offline installers and extras. You can do that through Galaxy (set the download folder to your new HDD in options) or using a browser. If you have hundreds of games you might consider something like gogrepo.py to batch download and update your offline collection.

Offline installers. Also try and source a 2nd backup drive. If one fails you can then just buy a new one and mirror from the other one without needing to re-download everything.

If you want to use the new HDD as a pure backup of your games, a storage, download the offlne installers. Via web or via Galaxy.

If you want to use the new HDD as your new folder for your previously installed library of games, this is, a transfer, then you can try copy and paste all your installed games folder there, but you do not need to copy Galaxy itself. Then Use Galaxy to revise/update/fix your games, pointing to that folder, to make the installatons functional and with all the dependencies and possible GOG fixes. During the scan It will only be done by Galaxy in the games neeed.

I also recently decided to back up my GOG installers. If you can manage to set it up, it's probably best done with GOGrepo, but it annoyingly coded in python 2 and I had some trouble getting that to run alongside the python 3 version I already had installed.

I ended up using Galaxy which turned out to be less annoying than one could think. You can just click on the game > extras > download all for the goodies + download the offline installer. Then you just click a bunch of games and wait...

You can also change the download folder in the settings so it directly downloads to your external harddrive which makes a lot of sense here. I would very much avoid trying to download the offline installers through the website.

The only thing that truly is a mess like this is keeping track of installer versions. If you don't right away download updated installers, you can easily lose track of what you should still update.

Uh? I installed Python 3 and I don't recall compatibility problems with Gogrepo.
Btw it's frankly essential to manage big collections, doing it manually is just unfeasible.

On a pretty good price it’s from wd elements on amazon

Here, check out the following thread.

https://www.gog.com/forum/general/do_download_managers_work_with_gog_and_if_so_which_one

Just bear in mind, that if you are going to back up with Galaxy, and not doing the Offline Installers option with it, then you will be needing to backup your install folders, which is doable but not recommended, if just for the increased size you are dealing with, and in the case of many games, the slowness to back up so many individual files. Lots of small files can take far longer than a few big ones, and really make your drives work hard and get hot.

Offline Installers are the best for archiving, especially as they take up far less space and are quicker to copy or move.

Like someone else suggested, don't rely on one backup drive either. Just imagine if it died, and they can die anytime, all that downloading to do again ... even worse if GOG had meanwhile closed its doors ... an unlikely option right now, but you shouldn't rule out the possibility, not if you care about your games.

Not sure what I did wrong then. Maybe I did not download the most up to date version? I thought GOGrepo was written in Python 2 and there are some formats and modules in there, that give errors in Python 3.

How come? For me gogrepo.py and gogrepoc.py (the Kalanyr branch) work just fine in python 3.

You are using the Kalanyr branch, right? It seems to be the most actively updated version at the moment. I am using the dev version (gogrepoc.py) as it has extra features compared to the master branch.

https://github.com/Kalanyr/gogrepoc/tree/dev

I usually only download a game when I want to play it, after that I make the checksum check during installation and I backup both the installers and the installed folder with a save file when I finish the game.
I try to avoid to buy a game too early and I don't usually update a game after I finish it.
I always use the website (I don't really like clients). I tried gog galaxy in the past but I used it only a few times every year and it was only for something like cloud save (I already do that on dropbox so I don't really need to do it also on gog galaxy).
After I changed my pc I decided to avoid to install galaxy to avoid to vaste my hard disk space on that.

Like others have said, gogrepo.py works fine with Python 3. That' s the old original version or the fork from over 2 years ago or more recent.

However, them aside, there is a new kid on the block, that doesn't require Python or any third party programs, gogcli.exe by Magnitus.

I've even created a GUI frontend for it, so you don't have to fiddle with the command-line.

The following will take you to both of those. Plus I have also developed GUIs for gogrepo.py.

https://www.gog.com/forum/general/gogcli_gui_frontend_downloader_and_validator

I noticed you can also download the offline installers with galaxy what is nice

Offline installers via web browser every time. Also, think up front how you will:
A) organise the storage
B) collect information on what is there

I don’t use gogrepo as it doesn’t fit my setup, and I want my setup to cover all items, not just gog. So I have:
[series][subseries][game name] [year][platform]
The. Subfolders for type, e.g store, rob etc., subfolders for documents, save, mods, patches etc.
In this way I consistently store everything in a way I can automate checking and moving.

Then there is what is there. A lot will use excel with a sheet with all metadata on. I do myself currently as I ditched collectorz.com. Am looking at playnite which, whilst mostly a launcher (hence useless) can get metadata and stores the information in a c# database (well, will do in v9), which makes it accessible from sql and c# which is good for my programmed checks.

Next up you will need more than one drive, there is nothing worse than spending hours organising, getting metadata, downloading mods etc. And then the drive fails. It happens, no drive is infallible. Have at least 2 or better 3 copies. I have 3* raid 5 boxes each with 4 drives in now. Raid is not a backup, it just helps with drive failure. The three copies are the backup (as is th olde external drives I have). Now this can get complicated later when you need to see what is updated, and for that I recommend freefilesync. And excellent free tool which compares drives and copies/moves/deletes only what is needed saving lots of time.

Links:
Freefilesync: https://freefilesync.org/
Playnite: https://playnite.link/
The raid boxes I use: TerraMaster-D5-300-External-Enclosure-Diskless

Some other discussions:
https://www.gog.com/forum/general/offline_backup_general_thread
https://www.gog.com/forum/general/questions_for_those_that_back_up_their_games
https://www.gog.com/forum/general/storage_and_organisation_discussion

Also be aware of:
https://www.gog.com/forum/general/provide_a_full_and_complete_changelogged_download_system
Gog fails to provide basic information on updates, change logs, patches for offline users, lack of decent download options eg language packs for large games.