I just purchased the DLC for cuphead, and I am new to gog. I have already purchased the main cuphead game last week. When I go to my account, under games, it lists cuphead, but I do not see the DLC. It has a link for the OST, and the link to install the whole game, but nothing for the DLC. Do I just click the "download and install now" button to install the whole game again? How do I install the DLC?

EDIT: I figured it out. Problem was that I had been playing off-line so I did not see the gog interface that comes up behind the game.

I do not understand. I have just purchased the Delicious Last Course DLC. The download is a 1.5 GB .PKG file, not an executable. What am I supposed to do with it? There are no instructions. I tried putting the file in the game directory, but see no difference in the game.

The post above doesn't help. I too have been playing the game offline up until now. Is being online required for the DLC? If so, there is no reason for requiring to be online unless it is some form of DRM, which GoG isn't supposed to have.

What is "the gog interface that comes behind the game", is it GoG Galaxy? I do not see any kind of extra interface or launcher for the game, just the usual shortcut that starts the normal game.

That's the deal: the 1 MB download wasn't appearing for me, just the 1.5 GB one. So I was wondering, maybe I have to change my user interface to Chinese for it to appear?

Also, GoG support doesn't work. When I click on support, the site moves from "gog.com" to "support.gog.com" and says I'm not logged in (but I am). When I click the login button, it just sends me back to the main "gog.com" site, where I am already logged in.

Finally, I solved the problem. I clicked the "platform: windows" selector, clicked on "Mac" (even though I don't own a Mac), and repeat again but select "Windows" once again. Only now the two downloads appear there: both the 1 MB one (part 1) and the 1.5 GB one (part 2).

I have no idea what caused the problem in the first place. Maybe I bought the DLC very soon after release, GoG forgot to include the first file, so only the second one appeared on my library. Later they fixed it by adding the other file, but my library wasn't fixed until I switched platforms – which I had to figure out by myself (after following a suggestive screenshot by 1083706812's post above – thanks!).

By the way, I found out now that the ".PKG" file is the Mac version. I'm absolutely positive I had "Windows" selected when I downloaded it though. Should have taken a screenshot at the time to prove it.

I can confirm. I just bought the DLC and went through the same scenario. GOG's download page was showing the Windows installation files for the base game, but the Apple version of the DLC. I followed the same steps mentioned above, switched OS setting to Apple then back to Windows and it displayed the proper two files for the DLC download. A very strange glitch. Thanks for sharing how you sorted it out as it was very helpful for me.

Below is the solution that was given to me by GOG support. A bit long and not easy to set up but it worked perfectly for me.
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Hello again,

It looks like I only got 26% complete (a very hard platformer!) and the DLC is already available so you should definitely have access.

I am suspecting that this might be an issue with your antivirus software causing some files to be damaged.

Please could you create a backup copy of your saved games (just to be on the safe side). These can by found by using the combination of WIN key+R and typing:
%APPDATA%\Cuphead

Next, please use the combination of keys WIN+R and type:
appwiz.cpl

Once here, please find the game on your list and double click it to remove it.

This, even should an error pop up, should remove any registry entries etc. for the game

Next please try removing any residual files:

Please go to (use the combination of the WIN + R keys and type):
%APPDATA%

And delete the entire Cuphead folder.

2) Please create a backup of you Windows regiestry (just to be on the safe side):
https://support.microsoft.com/en-us/topic/how-to-back-up-and-restore-the-registry-in-windows-855140ad-e318-2a13-2829-d428a2ab0692
the video below might be helpful (please make sure that the "Computer" entry is highlighted while doing this):
https://www.youtube.com/watch?v=S93_3j5Sj5s

Now please go to the registry entry:
HKEY_CURRENT_USER\Software\Studio MDHR

Please delete the entire Cuphead entry and all of the corresponding keys.

3) Lastly please make sure that the game folder is no longer present on your PC and if necessary, manually remove it.

Now restart GOG GALAXY and check whether you are able to reinstall the game.

Please reinstall the game AND add it to your anti-virus/firewall software's exception/trusted list.
Please make sure that you install the game to a short path, for example:
C:\GOG or C:\Games

The DLC should be now automatically added with the GOG GALAXY installation. I have went ahead and refreshed your purchase once again just in case.