Hello!
What kind of legal problem would somebody encounter, if he would create an open source remake of the chaos gate game?
If the remake would use the original files from the game that need to be purchased?
Or the game would be created with 3D graphics according to the actual standards?

Thanks in advance.

If you were to recode the game from scratch, but make it do basically the same thing (e.g. look almost the same, just perform better), it would be tolerable if you would have your open-source version install the art/sound files from the existing game. It's how OpenTTD and OpenRCT2 do it.

Remaking it in 3D... dunno.

To put it bluntly, Games Workshop hates anyone using the WH40k universe without paying them for a license first.
Ripped or remade content is right out.
Creating an engine replacement that uses the game's original data files and is nonfunctional without them might have a snowball's chance in hell, but only that much.