Okay, got Classic Rebirth + Seamless HD working with GOG version (I launch through Galaxy and it runs). The issue is indeed that the GOG version doesn't have the files required for the HD mod, as the HD mod relies on the JPN files and this is US. I can't go into details, but you have the right to get "creative" if you paid for this and want a better experience, so get creative.

For those of you who have problems using Seamless HD with this version of GOG, follow these steps "with the Japanese version downloaded from GOG":

https://www.youtube.com/watch?v=aN0oKsqdO14

The tutorial is in Spanish, but you don't need to understand the language, just pay attention to each of the steps that are taken. Many of the problems you have are still due to having to install a patch that Capcom released years ago for PC version in Japanese.

This works with Resident Evil on PC in its GOG version, but we will have to see if we will have problems with Resident Evil 2 and 3. It must be taken into account that Classic Rebirth was made thinking about the Sourcenext versions of Japan, if the GOG versions are not based on the Sourcenext versions we may have more than one problem when trying to use Classic Rebirth

Had that issue too. Had to download the japanese version (when you install it), then followed the steps EXACTLY.

Boot up game normal, save, close out.

Add specialized .exe, boot, save and close out

Add Rebirth, boot, save and close out

Add Seamless HD, Boot save and Close out. From then on it worked perfect.

haha, yeah, I'm dumb, I thought selecting language from the installer would just change text to that language in-game, not be an actual different version. Funny thing is, once you have the Japanese version installed through GOG and all the mods on, it doesn't have this "horr" folder either, but so far has been working fine.

With the Hiqh Quality Audio mod, do I use the Director's Cut or Dual Shock version?

The music in the Dual Shock Version is a hilarious mess. It was created by a bloke who pretended he was deaf.