You can get both the preorder bonus, and recent DLC. by manually creating two plain text files.
Files GOG uses for install management, the system originally created.

You can manually create this file from publicly available information, and the base games.
These are standard installer Info files, nothing special, or even uniqiue.

Pre-Order Filename
goggame-2125000806.info
Content DLC Filename
goggame-2002301113.info
Content Forgot to mention, if you use Galaxy. You may not know where the files go. It's where you install the game, so:
<Game Folder>/goggame-2125000806.info
<Game Folder>/goggame-2002301113.info

Steam might be different, but I doubt it, it's likely just figuring out the right text "Info" file.
I suspect the Games AppManifest_1731070.acf, as similar DLC Info gets added into that

This reminds me of Ubisofts similar dumb system, of adding Micro{Trash}actions, into the base game, and toggling them on/off in memory, which gets baked into a savegame.

This is worse, if this Basic GOG management file is present, DLC shows up, if not present no DLC, nothing else matters.

@ Devs
Want to sell added content, don't add it to the Base Game, you're giving away for free, what you're selling.

To be clear, I learned this because I wanted the Pro-order content, and couldn't buy it.
Then just had to test it out when your DLC released.
Maybe sell Pre-order bonus after release, and above all don't put DLC into base game, modders will find it sooner, or later.

Ubisoft games are the laughing stock of the modding, cheat engine communities, they've been doing this dumb thing for years in every game, and every Ubi{trash} game is rife with Micro{Trash}actions.

I gave up on buying Ubi{Trash} in 2019, and when they started their NFT Bullshit, I deleted my entire account, losing access to 17 games.

I have the moral right to crack those games I already paid for, but I don't, because my fangirl love for their games died. I only have the old games that are sold on GOG now.

I'm more tolerant of small dev teams

DLCs are artificially cut content scam in plain sight.

Not accurate, so let me clarify.
The DLC content was Added with the latest patch, which released alongside the DLC, but the content was patched into the base game.

It wasn't ALWAYS part of the base game, just the way it's been added is rather dumb.
Other one is a Pre-order Bonus, again not cut content left in the game to sell.

My investigation was triggered by NOT being able to buy the Preorder DLC.
Very Hard to scam you, by NOT selling such fluff.

This is nothing new. Lots of games on this site have tiny "5mb" DLC and expansion downloads. It's pretty clear in those cases that the content is already there in the base game download and simply locked until you install the "key" for it.

This is just the modern version of "disk-locked content".