Hi, I'm playing Undying using D3D in Windows 10, Nvidia driver 388.00.

I've gotten it working in 1920x1080 and proper FOV, in fullscreen. However, the in-game brightness slider doesn't seem to do anything.

Any ideas as to what's going on?

No.
But have you tried to use dgVoodoo2? It can solve a lot of rendering problems.

Thank you very much, that did the trick after some tinkering!

You're welcome, but thanks go to dege!
dgVoddoo2 is an extremely powerfull tool! Everyone should know about it!

In case anyone stumbles onto this thread while googling "smth smth Undying brightness" and dgVoodoo2 (or manually adjusting gamma through GPU drivers, for that matter) doesn't work for them for some reason: there's another workaround using Microsoft Application Compatibility Toolkit, with detailed instructions here
(copy of the thread on Wayback Machine).

Bear in mind: it does fix brightness slider, yet it also breaks vsync and I had no luck forcing it.

Posting here because apparently it can be applied to Unreal Engine games in general, also just for kicks.