Tweaks apply to any card and the hardware gamma should be available on most driver sets, only Nvidia forceware is explained here

High Quality Visual Tweaks
Disable vsync in the Geforce display properties and add the following to your Quake2 config files: 

set gl_bitdepth "32"
set gl_dynamic "1"
set gl_picmip "0"
set gl_playermip "0"
set gl_skymip "0"
set gl_round_down "0"
set gl_texturemode "GL_LINEAR_MIPMAP_LINEAR"
set gl_ztrick "0"

NOTE: You might need want to change intensity to 2 on some displays to improve the whites.



Brightness and Colour
To get the 'Voodoo' colour range in Quake2 with a Geforce/TNT do not use palette hacks, drastic gl_modulate, intensity etc changes. Simply save a preset called 'Quake2' under the Nvidia display control panel 'Colour Correction' with a gamma of between 1.80 and 2.30, be sure to also save a preset called 'default' with gamma 1.0 as well. Then enable the Nvidia 'taskbar utility' to allow you to quickly swap between gamma presets. 

Before starting Quake2 select the 'Quake2' gamma preset, your Quake2 will then look the same as it did with the Voodoo range of cards without any detail or colour saturation problems caused by adjusted palettes or drastic changes to gl_modulate, intensity etc. Once you exit Quake2 select the 'default' preset to return to standard gamma settings. 

Image taken from on the multiplayer map 'The Edge' with hardware gamma correction used and default Quake2 gl_modulate, etc. settings. You might need want to change intensity to 2 on some displays to improve the  whites.