What download managers are compatible with GOG? I've used JDownloader for a good while for other things, but now I want to do backup my GOG offline installers, but it doesn't seem to be compatible.

You might want to look into gogrepo.py. Takes a bit of tinkering, though.
https://www.gog.com/forum/general/gogrepopy_python_script_for_regularly_backing_up_your_purchased_gog_collection_for_full_offline_e

I will check it out, cheers.

Galaxy functions as a downloader for the offline installers now, in case you didn't know.

I have been using Free Download Manager 5 for a good long while with GOG ... works well.

I've also created a GUI (frontend) for the gogrepo.py script, and used that a good bit.

It's called GOGRepo GUI and can be checked out here.

I've also kind of combined those two programs with my GOGRepo Simple GUI program, which you can check here.

If you struggle with or don't want to deal with the gogrepo.py Python script, you could use another one of my programs with Free Download Manager 5, which is called GOGPlus Download Checker. Details found here.

My GOGRepo GUI does the full download and checksum thing etc, plus queued downloading, and one click for all the files related to the selected game. It however needs Python & some libraries installed, plus the gogrepo.py script.

But Free Download Manager 5 in combo with one of my (those two) other programs is also a great solution. It takes a few more clicks but also does queuing. My programs test the integrity of the downloads, checksums etc. You also gain the benefit of being able to set the number of threads with FDM5 and download speed ... not sure about resume, but probably.

Free Downloadmanager doesn't need additional "scripts". I just use developer options from Firefox to copy the link from the GET request. A little awkward, but it works. jDownloader can certainly do this on its own, but I'm actually too suspicious of it in every respect.

Good necro.

I now recommend using gogcli.exe if the Python based gogrepo.py is not attractive to you.

As for Free Download Manager 5, which still works well, and which I use to overcome poor single thread download speeds with GOG right now, there is a browser extension (addon) for it, that lets you click links to add them to FDM5. I personally have that set to work with the ALT key, held down while I click the links. No need to muck around with Developer options.

P.S. I've also recently had some success using curl.exe to download my GOG game files, using the browser link, which I can also get from the manifest in my GUI for gogcli.exe. I've even come up with a type of multi-thread like process using curl, which is a single thread program. It does, as my good friend TheDcoder says, hammer your drive a bit, as you are essentially doing multiple simultaneous downloads of different parts of a game file and then using the Copy /B process to join those separate files together afterward. I've tested that with the MD5 values with success. Basically you are working with a known amount of bytes for a game file, and then divide that up with some math, and then using curl parameters to get the different sections of the game file. The curl program also has a resume ability. Using curl in such a manner enabled me to download a game file much faster. I did try to use aria2, which works for some folks, but won't work for me on this PC, and has multi-threading ability built in. Both curl and aria2 require a cookie file.