I tried making the whole steps to make Classic Rebirth+Seamless HD Project run on the GOG release again yesterday, and I had good success with it in a very clean way, so I am currently playing with CR+SHDP without issues under Linux on a Steam Deck. I even managed to unlock Hard Mode and all the Special options right from the get-go, similar to the GOG but with the SourceNext conversion.

Here's a step by step guide on how to accomplish it. I lack proper images or screenshots of the process since I did it as a casual thing yesterday on my own.
Do note that I made all the testing and process originally on a Steam Deck (other Linux systems might work similarly too), so steps might vary for Windows users:

1. [Steam Deck] Download and install the Heroic Launcher and link your GOG account to it.
2. Install Resident Evil 2 (1998) from GOG in your system
3. [Steam Deck] Once RE2 is installed, go to the game's "Settings", and go to the "Advanced" option. Here under environment variables, add the following:
   WINEDLLOVERRIDES="d3d9,d3dcompiler_47,ddraw,dinput8,dsound,libwebp,xaudio2_9=n,b"
4. [Steam Deck] If you are instead running the RE2 EXE directly on Steam as a non-Steam game, then inside Steam settings for the game add the following line under "Launch Options":
   WINEDLLOVERRIDES="d3d9,d3dcompiler_47,ddraw,dinput8,dsound,libwebp,xaudio2_9=n,b" %command%
5. Download the "Unofficial SourceNext 1.1.0 Conversion Patch for RE2 Windows 95-98" mod from NexusMods (Archived link)
6. Extract the contents of the patch's ZIP, and then copy over all the folders inside the "SourceNext" folder (Common, Pl0, Pl1, Zmovie, savedata) into your game's installation folder (where the RE2Launcher.exe file is). It should prompt you to overwrite the files and folders, do so and overwrite all files. If you didn't get an overwrite prompt, the files were not copied in the proper folder. The SourceNext folder is the only thing we need from this mod, the rest can be ignored.
7. Copy over the CD-Key.txt file inside the SourceNext conversion Zip, this will be used later.
8. Inside the game's installation folder, copy both the "ddraw.dll" file and the "RE2Launcher.exe" file. Rename the copies to "ddrawR.dll" and "RE2LauncherR.exe". This just so you can have a backup of the original files and are able to launch into the original GOG release if desired later on.
9. Download the Resident Evil 2 Classic Rebith patch, as well as the SourceNext 1.1.0 official patch.
10. Extract both patches, and copy over the "ddraw.dll" file from the Classic Rebirth patch over to the game's installation folder (should prompt to overwrite), and for the SourceNext 1.1.0 update patch, rename the "bio2 1.10.exe" file to "RE2Launcher.exe", then copy it over to the game's installation folder and replace the existing file.
11. Once all files are copied over, try to launch the game (or through Steam if on Steam Deck). It should open up the Classic Rebirth configuration/settings window.
    Enable or disable whichever options you desire (enable Quick Turn for testing), just make sure that you have the option "Color" set to "RGB888" for compatibility with the Seamless HD Project mod.
12. While the Classic Rebirth Settings window is still open, go back to where you copied the CD-Key.txt file from Step #7 and copy the key, paste it in the textbox inside the Classic Rebirth settings window (the one above the "Legacy framerate" option), and then click on OK.
13. The game should launch properly if the setup was done correctly. Get in game, and press Down+Run to test if the Quick Turn works, if it does work, then Classic Rebirth is working properly with your RE2 installation.
    
    
14. Once the game has launched successfully with Classic Rebirth, a new "config.ini" file will be created inside the game's installation folder. Open it, and modify the following parameters with these matching
    1. GALLERY0 = 0xFFFFFFFF
    2. GALLERY1 = 0xFFFFFFFF
    3. GALLERY2 = 0xFFFFFFFF
    4. GALLERY3 = 0xFFFFFFFF
    5. GALLERY4 = 0xFFFFFFFF
    6. SPECIAL = 0xA
    7. SuperHardSwitch = 1
       
       These changes will make all the stuff under "Special" to be unlocked by default, just like the GOG release does, but ported over to the SourceNext version.

Now if you want to apply other Quality of Life mods, follow these next steps:

• For the High Quality Sound Mod by lexas87:
  Download the package from Mega, extract all its contents into a separate folder. For Windows users, you can copy-paste all the contents directly into your installation folder without any issues, you should be prompted to overwrite the contents straight away.
  However, for Linux or Steam Deck users, be absolutely sure that each of the files and folders match the naming convention that the GOG installation uses, it is absolutely important that the folders and files match letter by letter, since Linux systems and the Steam Deck are case sensitive when it comes to file/folder names, and even the slightest of change in an upper or lower case letter will create a second file or folder if they don't match.
  Once you have checked/modified the files and folders and they match precisely, copy-paste the contents into the game's installation folder.
  NOTE: Using this mod changes the title's "Resident Eeeeeevil 2" voice to be that of the Gamecube release. If you want the original voice line instead, go to Common/Sound/core/ and delete the CORE17.SAP file, and rename the "CORE17.SAP.BAK" file to "CORE17.SAP". The BAK file is the sound with the original voiceline, so if you want that voiceline, we want to be the main core17.asp file instead of the one provided by default.
  
  
• For the MP4 movie replacement Mod by Reza Molaee:
  Download the package, extract the files and copy them over to the game's installation folder. You should be prompted to overwrite.
  Be aware, as while this mod brings better quality FMVs, it also adds Japanese subtitles and these cannot be removed.
  As far as I'm aware, this pack doesn't have any case sensitive issues for users on Linux / Steam Deck, but double check to be sure.
  
  
• For the Seamless HD Project Mod:
  You need to download TeamX's RE2 HD mod first, make sure to download the mod from ModDB (around 1.8GB), and extract from this pack ONLY the following three files: bio2hd.asi, dsound.dll and libwebp.dll. Without these files, the HD textures won't load into the game! The rest of the folders from the pack can be ignored, though it you want to, you can keep the Pl0, Pl1 and Zmovie folders, since those offer really high quality FMVs (albeit with Japanese subtitles still).
  Next, download the main Seamless HD Project mod from ModDB (around 700+MB), and extract the "hires" folder in the game's installation folder. Then download the bugfix uploded later to display the Flamethrower and Sparkshot ammo correctly from ModDB too and extract it over the hires folder from SHDP.

If setup correctly, then you will have the RE2 GOG modified to use the SourceNext EXE, with the unlockables from the GOG added into the SourceNext one by default, and also with full Classic Rebirth, HQ audio and Seamless HD Project installed into it as well for the ultimate RE2 experience. (Even though Classic Rebirth is missing the EX Files from the N64 update )

 

[PROTOBOY]

Does this mode changes the title screen for the sourcenext one?? I mean this title screen where we can select the characters??

Or still I would have to access each scenario trough LEON.EXE/CLAIRE.EXE like the WIN 95 version?

 

[ShadowOne333]

Does this mode changes the title screen for the sourcenext one?? I mean this title screen where we can select the characters??

View attachment 455570

Or still I would have to access each scenario trough LEON.EXE/CLAIRE.EXE like the WIN 95 version?

Yeah it changes it to the SourceNext one.

If you replace the RE2Launcher.exe file with the bio 1.10.exe from SourceNext, you can select the character in-game.

That's one of the reasons why I decided not to use the ClaireU.exe and LeonU.exe files from the Unofficial conversion patch, because those lock you into using the PC Window menu to select the character instead of being able to select it in-game.

 

[xst8799]

Thanks for the guide. I was able to make it work finally!
But I can't figure out how to make it work in full screen. It only opens in a small window, which isn't a very ideal to play.

 

[ShadowOne333]

Thanks for the guide. I was able to make it work finally!
But I can't figure out how to make it work in full screen. It only opens in a small window, which isn't a very ideal to play.

Probably you need to adjust the settings.
Check the config.ini or try to change the resolution from the Classic Rebirth options window that opens up when you boot the game.

If it doesn't appear, then open the config.ini in a text editor and change the option that says something about the menu from 1 to 0 so it opens up. Or change the resolution in that same file.