Is there downloader program for easier backup of the GOG library? "Game goodies" have to be downloaded to a separate folder and games & extras must be kept up to date, preferably multi-platform. Verifying file integrity for all files is a major feature.

To my knowledge GOG provides no batch download functionality.
Sorry if a thread already exists, it is very difficult to search as the keywords are so common.

I've found an informative thread: https://www.gog.com/forum/general/is_it_possible_download_gog_games_from_the_command_line

This seems to be a definitive solution. I have to use the tool for a while and see if "better" solutions have been posted, then I will consider accepting your answer.

Not found a tool yet that downloads what I want, stores it where and how I want. Sure there is gogrepo.
Me I download everything as I buy it, store it myself into a given folder structure, add the relevant data about it to a database, and then mirror across to other drives. An automatic process could overwrite a version with a broken patch or might contain language updates or something else not needed, benece why I don't find automation that great, although I have tools to create and check library structure. The thing with it is though, you really need to be pro-active about it straight up, otherwise it becomes a lot of work even if you programmatically create and alter file structures.
One day I will get round to writing my own database/tool to do all the above.

One day... just not today as it is such a lovely summer day today.

A list of download clients can be found on gogwiki
https://www.gogwiki.com/wiki/Download_Clients
Although most of the clients on the list aren't working anymore and are not maintained

There are only 3 working clients at the moment that I know of


LGOGDownloader
https://www.gog.com/forum/general/lgogdownloader_gogdownloader_for_linux/page1
https://github.com/Sude-/lgogdownloader

gogrepo.py
https://www.gog.com/forum/general/gogrepopy_python_script_for_regularly_backing_up_your_purchased_gog_collection_for_full_offline_e/page1
https://github.com/eddie3/gogrepo
Kalanyr's fork: https://github.com/Kalanyr/gogrepo/tree/working2

GoodOfflineGames
https://www.gog.com/forum/general/html_frontend_to_your_offline_games_collection/page1
https://github.com/boggydigital/GoodOfflineGames.exe

gogrepo does not download extras to a separate folder, but to the same folder where it downloads the main installer files as well. Not sure why that is important to you that they are separated?

If I recall right, gogrepo lets you choose whether you download only the main installer files. or the extras, or both. So I guess you could run the download in two passes, first downloading only the main installer files, and then a second time downloading only the extras to a different place.

It is a great tool to keep your local installer collection up to date, though. I use it regularly.

I've always preferred to separate extras from the installer files as large games can have a dozen installation files.

That is one reason why I don't use automated ones. I have various folders with each game folder, for instance:
Docs, saves, mods, patches, engines, source etc.
I also have comics/books removed from that area to my books area.
The structure then becomes more complex when multiple versions of a game are needed to be kept. This also has to be linked to a DB.
So it's not just a simple download latest of everything to a folder.

How many games do you have on GOG? With 1384 games. I just don't have time nor capacity to do that kind of micromanagement. Also, tracking (and downloading manually) updates to games is just too hard with this many games. I tried that for awhile back when I had something like 500-600 GOG games, and already back then it was an overwhelming task, quite soon I lost track which games were not up to date.

To me it is enough that my GOG game installers and extras are in somewhat understandable folder structure so that I can find the title I am looking for pretty easily from my hard drive. It is not like a buttefly or a stamp collection where everything has to be in perfect order, it is simply an offline storaga place for my GOG game installers and related files.

EDIT: About save games, if I feel I want to keep them after finishing and uninstalling a game, I have a completely separate folder structure for those (also non-GOG games). I don't keep them with my GOG installers. Same for any mods, non-GOG updates and such.

Nowadays quite often I don't keep old saves anymore though. I feel that if I want to get back to a game later (after finishing it), I will replay it from the very start anyway.

On GOG, yes around the same as you. Overall, in the 10k range, not all digital, not all games (could just be patches and things). Hence why getting it right with consistent structure is so important. I can write code to proces this fixed structure, check integrity and such like. A year or so back I migrated into a new structure, ran various checks and moved from excel to a database, this took 3 months even with automation, and this didn't include spectrum roms. Gogrepo might work if gog was your only store, but I think most people use several, even without steam there is 20-30 stores or dev sites I use, gogrepo doesn't work there.
Not getting into book, music and other files management, it's amazing what digital junk you build up over your lifetime, and I am a hoarder, even programs from back in the early spectrum days!