Tafferpatcher installs NewDark, which improves compatibility on modern systems. Also contains mission fixes and optional texture mods. The GOG release has an out-of-date, stripped-down version of this patch.
The Enhancement Pack 2.0 is an updated version of the original Enhancement Pack (which is included in Tafferpatcher and is much less comprehensive). This contains higher quality reproductions of original textures and objects. Also works with Thief Gold/The Dark Project.
The Necro Age pack is very similar to the Enhancement Pack 2, but focuses on giving the game a slightly darker, noir look. Somewhere between the EP2 and HD Mod in lore-accuracy. Also works with Thief Gold/The Dark Project.
The Thief 2 HD Texture Mod adds new textures and effects to all original missions. Includes a tool to configure advanced graphics settings (Anti-Aliasing, Bloom, etc.).
Install NewDark via Tafferpatcher.
If you still can't change gamma:
This mod provides cleaned up and resampled sound files for Thief Gold and Thief II. The quality difference is particularly noticeable when wearing headphones.