I left it to download over night, it was broke in the morning.
Probably my router switched IP and that wasted 15G. (I used browser, not dl manager)

Advice to anyone with dynamic IP; restart router before initiating this dl.
I wish I had thought of that yesterday.

Use GOG's Downloader for this, if you need to resume downloads.

Thanks, but I don't need it. It broke again at some point, during the day, but renaming the .part file fixed it.
I just needed to start new dl, pause dl, replace the new .part with the old 15G one and then resume.

Long story short:

It depends on 32bit SDL2_image which doesn't compile (SDL2 compiles, but not SDL2_image)
I've no time to dig into this any further so I removed the package.

Maybe some other time.