What are the differences with this version? https://www.gog.com/game/corpse_party
Taken from one of the devs on Steam:
The old Steam version is from the original Corpse Party devs, an indie studio called GrisGris.
-Indie art by GrisGris, the original devs.
-Indie voice acting by different voice actors than later games/releases. Voice acting is a little spotty in early chapters
-No CG artwork for Wrong Ends and many other scenes, descriptive text instead
-4 Extra Chapters, including "Tooth" presented in RPG style instead of Book of Shadows style
-A couple of different/extra endings in Chapter 5, if I remember right
-A couple of GREAT vocal songs that are completely missing from later versions.
-Steam version is available for Windows and Linux.
The 2021 version is from mainstream developer MAGES.
-Art by MAGES. which matches Book of Shadows, Blood Drive, etc.
-More-or-less full voice acting by the same actors as Book of Shadows, Blood Drive, etc.
-Much more of the voice acting is binaural.
-Redone area and sprite graphics at 1.5x the resolution
-CG artwork for many scenes and pretty much all Wrong Ends
-16 Extra Chapters (Tooth is not one of them, but the other 3 from the old version are there)
-An Omake "Chapter" where you can view many of the CGs
-An extra Wrong End in Chapter 1
-An opening video
-Different (not remastered) music for a few scenes or locations
-A couple of its own unique (and very good) vocal songs
-Steam version is available for Windows, Linux, and also Mac.
To add to this, it's a port of the handheld remakes of the original game that weren't available up until this point and brings the production quality in line with the sequels already available on GOG. It being a port is a good thing, because while the series started as a PC game it was less polished. As a general rule, the Japanese PC gaming scene has always been much smaller than their console market and it's the console version of the game that'd get a beefier release. This release appropriately has quite a bit higher production values.
I can't afford buying this version myself, but I'd be surprised if my original review for the older version doesn't apply to it. This should be the definitive Corpse Party that you'll want to buy if you're interested in the series. Per my review of the original, the game itself is still a pretty effective little horror game and a cult classic and now the missing handheld content is included.