I'm downloading The Witcher 3 GOTY 1.31 right now (classic game installers and Polish language packs).
Where can I find MD5 hashes for GOG's files to confirm that my downloads will be ok?

EDIT: Here is the list of hashes for other readers. My thanks go to kbnrylaec, Starkrun and Midoryu. :)

321ed8cc0faedb903190a708686a1b50 *setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709).exe
8aab6124c22f1360585ee2285ea6d8f7 *setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709)-1.bin
3966e84f941b9fba384eb4a2b0b23c65 *setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709)-2.bin
bc80aa6b1538ecf757e7db6f3723e056 *setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709)-3.bin
71660541a0c358ef40802ba62a2c3c09 *setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709)-4.bin
9ad56c2efc7b09f480f7f8c7922c8b3f *setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709)-5.bin
bd3699654b2e34668445219f2bbbc793 *setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709)-6.bin
1beb5a622e6695d0dd65cac5fab08793 *setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709)-7.bin
121adc43111562e354d9841800d0c613 *setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709)-8.bin
0f5329306515d6f41a1b4a7b2d38ad74 *setup_the_witcher_3_wild_hunt_goty_1.31_(a)_(9709)-9.bin
ba8d5d02f2fd132a4aa94921b869682b *patch_witcher3_pl_goty_2.0.0.51.exe
0464d5f235634baa0fc64ad9d7836bbe *patch_witcher3_pl_goty_2.0.0.51-1.bin
4263d8532d2e48b8e4c4cc6689bdb34f *patch_witcher3_pl_goty_2.0.0.51-2.bin
d45401fe884b5cf6ca99ecd15d4e8673 *patch_witcher3_pl_goty_2.0.0.51-3.bin

Short answer:
gogrepo.py and lgogdownloader will automatically check the md5sum for you.

Long answer:
GOG usually provide a xml for each offline installer file.
For example:
https://www.gog.com/downlink/dragons_dogma_dark_arisen/jp1installer2
download it and you will get:
(some URL here)/setup_ddda_japanese_1.0.0.18_(12573)-2.bin?(a quite long string here)

The xml for it is: (append .xml in the middle)
(some URL here)/setup_ddda_japanese_1.0.0.18_(12573)-2.bin.xml?(a quite long string here)

The xml look like this: NOTE:
1. some xml files are missing, but the situation is rare
2. GAME GOODIES files have no xml files

Thank you for the answer. I'm downloading in normal way, without any additional downloaders. I'm trying to use your method and I can see quite a lot of chunks / md5s in this xml file.
Do I suppose right that the proper one is the first one, after "notavailablemsg" tag?

Only the first line is necessary.
Rest contents are only useful for geeks and automatic tools.

From my heart, thank you. I didn't know such a file existed and I have always downloaded the same file twice to verify the integrity of the installers.

I've just tried it with a few installers and it is very helpful :-)

I have the GOTY, 1.31 (a) installers here are the MD5 hashes from my verified downloads. I use gogrepo.py for archival purposes and do a full archive check via checksums every month. My GOTY has never failed its checks.

Thank you very much for these hashes - just started verifying my downloads and they seem to work with them. :) Well, it seems that Witcher 3's Polish language packs do not have these... :(

Has anyone got access to verified Polish language packs and can prepare MD5 hashes for me to check?

MD5 hashes for the Polish language files downloaded via GOG Downloader, not Galaxy:

patch_witcher3_pl_goty_2.0.0.51.exe
ba8d5d02f2fd132a4aa94921b869682b
(edited from wrong value 09579524c5b9efbb56050ee84670749c)

patch_witcher3_pl_goty_2.0.0.51-1.bin
0464d5f235634baa0fc64ad9d7836bbe

patch_witcher3_pl_goty_2.0.0.51-2.bin
4263d8532d2e48b8e4c4cc6689bdb34f

patch_witcher3_pl_goty_2.0.0.51-3.bin
d45401fe884b5cf6ca99ecd15d4e8673

( I used WinMD5 to create these, downloaded at http://winmd5.com )
_________________________________________________
Edited a day later: Fixed wrong value thanks to Lexor.

Here is a wishlist entry for GOG to add checksums to the game shelf.

Thank you very much for these! <3

But I have some problem / error with just exe file (used Total Commander to check) - I've downloaded this file a few times, the problem did not repair. Are you sure about your hash for this file? My hash is: voted +1 :)

Sometimes GOG provides different files that have exactly the same name.
For example, dying light 1.12.2_(12711) have at least two different sets of files.
They have the same filename but with different filesize and md5sum.

So, always compare the md5sum with the latest xml file.

Well, the problem is: this file does not have xml file as I mentioned in one of my posts above:
https://www.gog.com/forum/general/md5_hashes_for_big_downloads_like_the_witcher_3/post7

You're right about the hash. It is ba8d5d02f2fd132a4aa94921b869682b indeed.
No clue where I've got the other value from, I apologize for the inconvenience I may have caused you.

Edited 5 minutes later:
Hah! I've found my error! The wrong hash belongs to witcher3_lang_pl_2.0.0.52.exe, which is the file in my backup folder downloaded by Galaxy over a year ago. Originally I started to make a list of all the hashes, only in the middle of it I realized these were of no use for Lexor and downloaded the files again through the website. That's why I edited the list from scratch and copied the hash wrong and/or didn't paste correctly. Something like that.
*Phew* This would have driven me mad. Not knowing why, that is.

Thank you for this post. :)

Do you know if gogrepo can also create .md5 files to store next to the downloaded installers?