Thought it would be a good idea to assemble a topic with the most common issues and their resolution. Feel free to post yours along with the solution you have found anywhere on the internet, if you can reproduce the issue and have an idea how to fix it, even better.

Q.: I bought the retail version/collector's edition, what happens if the disks are damaged or I lose them?
A.: You will find a code inside your boxed copy you can redeem through either the website or Galaxy and download the game whenever you like. You can find it on the back of the game manual.

Q.: Upon installing the retail copy, the setup is asking for a .bin file that is nowhere to be found.
A.: Usually a simple reboot solves this issue. If not, this is going to be a tricky one, those files should have been named differently on the DVDs, and the only way to change those is to copy everything to your hard drive, make sure everything is in one folder (Thanks Thiev and Unkalibriert)

Rename them as follows:

Disc 1:
SETUP-TH.EXE -> SETUP-TH.EXE
SETUP-TH.BIN -> SETUP-TH-1.BIN
SETUP-T2.BIN -> SETUP-TH-2.BIN

Disc 2:
SETUP-TH.BIN -> SETUP-TH-3.BIN
SETUP-T2.BIN -> SETUP-TH-4.BIN

Disc 3:
SETUP-TH.BIN -> SETUP-TH-5.BIN
SETUP-T2.BIN -> SETUP-TH-6.BIN

Disc 4:
SETUP-TH.BIN -> SETUP-TH-7.BIN
SETUP-T2.BIN -> SETUP-TH-8.BIN

OR if there is only one .bin on Disc 3 and 4:

Disc 3:
SETUP-TH.BIN -> SETUP-TH-5.BIN

Disc 4:
SETUP-TH.BIN -> SETUP-TH-6.BIN

Q.: I can't seem to activate the free DLC content on Galaxy.
A.: Check this thread here by Oksoy for a quick guide. You can also download them with GoG Downloader or directly from the website, it has been added recently.

Q.: There is no *insert language* in the game by default.
A.: You have to download the designated language packs first.

Q.: When I start the game nothing happens, but I can see the application running in the background.
A.: There has been a solution circling around that worked for quite a few people, thanks to Hycinthus:

1. Uninstall Galaxy
2. Open command line (Start->type "cmd" -> right click "cmd.exe" and click "run as administrator")
3. Type sc delete GalaxyClientService
sc delete GalaxyCommunication
4. Go to //gamedir\bin\x64 and rename or delete galaxy.dll
5. Launch the game, an error message will pop up, but the next time you start it everything should be fine

Q.: The files I downloaded are seemingly corrupted.
A.: Compare the checksums, if they don't match it is best if you re-download them.
MD5 hashes
b0fd11aac60f206011d8170f8a11c889 setup_the_witcher_3_wild_hunt_2.0.0.28-1.bin
da3a560ca2d93bc671682a9d916f968f setup_the_witcher_3_wild_hunt_2.0.0.28-2.bin
0dce528d1e5abcbcbb6e6d8f98bbeb9c setup_the_witcher_3_wild_hunt_2.0.0.28-3.bin
b73ff870a5103f49d8c5e3ecda1dd042 setup_the_witcher_3_wild_hunt_2.0.0.28-4.bin
8ee5cec54a0f42a8c75ae95db1b27508 setup_the_witcher_3_wild_hunt_2.0.0.28-5.bin
136f896662d8bd1f072c4fcd97c19138 setup_the_witcher_3_wild_hunt_2.0.0.28-6.bin
172a203132a2f348811f4c11a4b5063b setup_the_witcher_3_wild_hunt_2.0.0.28.exe
SHA-1 (courtesy of green.anger)
7298A1B16B0FE8E95C6C4867483A8BAE4225C007 setup_the_witcher_3_wild_hunt_2.0.0.28-1.bin
7A5AC11F2AB4B3617F47D490D989978ABA3C149B setup_the_witcher_3_wild_hunt_2.0.0.28-2.bin
A5033A733369049515D58639748AA3465C0815F7 setup_the_witcher_3_wild_hunt_2.0.0.28-3.bin
B270683B14B7ECA9A019FBAD4161AAB2D2D6E853 setup_the_witcher_3_wild_hunt_2.0.0.28-4.bin
E1E5672248265DE0DCAAB4948D7A5A61F767FC3E setup_the_witcher_3_wild_hunt_2.0.0.28-5.bin
98024B3DFA1EAA8E0476FE92AE8CBBFE1B3D7D07 setup_the_witcher_3_wild_hunt_2.0.0.28-6.bin
513845906EC9D4EB4C11B8265EB6ECE9ED1CA925 setup_the_witcher_3_wild_hunt_2.0.0.28.exe

Q.: I cannot bind actions to certain keys or they don't work as intended.
A.: Good news is you can change certain keybinds, unfortunately some are hardcoded.

1. Exit the game
2. Go to //documents\The Witcher 3
3. Open input.settings
4. Let's say you want to bind interaction to F instead of E
5. Open notepad, select 'Rename' from the 'Edit' tab and change every variable from IK_E to IK_F
6. Save and run the game, then realize you still have to loot with the E key :)

The only way to bypass hardcoded binds are using a third-party software at this point. is a free scripting program, here's [url=https://www.gog.com/forum/the_witcher_3_wild_hunt/a_quick_autohotkey_guide_to_those_who_struggle_with_remapping_keys]a quick guide how to change your layout.

Q.: Game is too dark, I'm scared, and I cannot find the gamma slider. I swear I have seen it somewhere before!
A.: You can only access the gamma slider if you have the game in full screen mode. Alternatively you can always go to //documents\The Witcher 3, open user.settings and change the GammaValue variable (though since you are in windowed mode, the game automatically uses your desktop's default gamma settings). For a more thorough explanation find green.anger's post about gamma customization.

Q:. Graphics settings seem to reset every time I start the game.
A.: Geforce Experience automatically optimizes your game upon start, try disabling it for TW3.

Q.: The audio level is very low or missing when the game starts, during dialogs and cutscenes.
A.: The only solution up to date is to change your configuration to either Stereo (2.0) or Quadraphonic (4.0) until the game is fixed. (Cheers Red_Eagle_LXIX)

Q.: I cannot import my saves from The Witcher 2, even though the save games are there.
A.: Certain games (TW3 included) cannot handle special characters such as diacritics in your username, so what we do is:

1. Create a new user with administrator privileges
2. Import save on that user
3. Save the game
4. Move the saves folder back your default user directory

Thanks ufoman

Q.: How to get a generic controller to work with the game?
A.:

1. Download the emulator
2. Copy files in //gamedir\bin\x64
3. Overwrite xinput1_3.dll
4. Use your controller to play

Thanks TitoOliveira

Q.: Upon installing the DLC packages my whole TW3 installation was deleted.
A.: You can read about the story and the workaround is in the stickies. (good find, merchantskeleton)

Q.: I pre-loaded the game, but upon trying to install the patch, Galaxy cannot find my game. I cannot patch it manually either.
A.:Best solution up to now is in this thread by cklim78, takes a bit of registry editing, which is done the following way:

1. Open the Start menu
2. Enter regedit into the command line
3. Go to HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\GOG.com\Games\1207664643

Here you have to manually edit the values to make it look like the one in the post. You still need to make sure that all paths are correct, so don't use "K:\\Games\\The Witcher 3 Wild Hunt\\" for your savegame folder if they are on C:\

Q.: The game doesn't support 1280x720, is there a workaround?
A.: It does as of the 1.04 patch.

Q.: Come on man, 1280x720 is for the high rollers. Can I make it work in 480p?
A.: You would have to access either Catalyst Control Centre or NVidia Control Panel to manually set the desktop resolution to 640 or 720x480 first, then

1. Go to //documents\The Witcher 3, open user.settings
2. Set 'Resolution' to 640x480
3. Set 'FullScreenMode' to 1
4. Save the file, close it
5. Run the game in 480p

Thanks fjdgshdkeavd.

Q.: I'd love to try HairWorks but I have an AMD card. Is there anything I could do?
A.: Yes, thanks to the lads on reddit.

Q.: I installed the game and it is well after 19th May, Galaxy still displays 'Waiting for release' and the 'Play' button is greyed out. What can I do?
A.:

1. Exit Galaxy
2. Open it up again
3. Select the game, click More -> Manage -> Verify/Repair
4. Wait for the patching process to finish
5. Play

Q.: Upon installing the retail copy and redeeming the code on the website, the game fails to update through Galaxy with the status set to NOT AUTHORIZED.
A.: You need to 'purchase' the free DLC program first, once that is installed it should automatically go on with the updating process.

Q.: I cannot seem to craft more than one alchemy supply or higher level mutagens since the latest patch.
A.: A known bug that has been reported already. The workaround is quite simple:

1. Drop the existing alchemy supply or mutagen
2. Craft another
3. Exit the crafting interface, loot the item you dropped
4. Repeat if necessary