There are a handful of supported DX12 games. Some that I know of: RDR2, Cyberpunk, Deus EX MD. But they’re either flat or Z3D.
For games that support mods there is a workaround to get G3D by using AFR (alternate frame rendering) where the mod changes camera position on each frame to match each eye. But since the game does not stop between frames each eye will see a slightly different scene (unless the mod can also hack timing). Still it’s a lot of work for each game. RDR2 is probably the only DX12 title with such a mod.
Other than that it might be possible if the game itself has built-in SBS support. However there is no SBS profile for DX12 that I know of.
So there is quite a lot of effort going into DX12. However my understanding is that G3D is not possible (at least not in the same way as for DX9/10/11). But you can hook into DX12 games and at the very least you get a flat screen in VR. For some you’ll also get Z3D or AFR (and maybe SBS in the future).