Unfortunately it looks like GOG did not package it up correctly. Probably going to take more time for Larian to help them get it right.
It's not GOG's job to package it "Correctly" that's on Larian.
If the Data DLC is added to the engine it's Larian's job to put it there to start with.
If as I suspect, it needs adding to the games Data folder, it's Larian's job to put it there.
Since Galaxy came out, the dev has the power to add content directly to the game.
GOG's only responsible for making the offline installer's, from the Galaxy files, which usually takes just a few hours longer.
Even before Galaxy arrived, or if there's a SNAFU now, the dev and/or Publisher is responsible for making sure the files are getting to the owners.
It's their game, and they are the ones responsible for getting it right.
GOG, and Steam are stores providing updated content for 1000's of games, while the dev is only usually updating their latest game.
I don't care whose to blame for the SNAFU, I still expect the devs to find out why, and get it fixed ASAP.
If that means providing it directly, or using a site like Nexus to host it for them, until the SNAFU's corrected, then do it.
Don't say "We'll discuss it", get it done. ASAP, and by anu means necessary.
This "We gave it to GOG, it's their fault", blame game, fixes nothing. That's just a lame excuse for inaction.
The tools are not even part of the game, and there's absolutely no reason to rely on GOG, or Steam to provide them.
Using them as added sources is fine, relying on them, for free tools is not.
Tools can be hosted anywhere, at anytime, and should even be available to everybody, not just owners, on that store.