GTA2 runs in DirectX 6 by default with a hard-coded 16-bit color depth, but also has the 3dfx glide renderer disabled by default. This program includes a glide rendering mode that, when using a Glide to OpenGL wrapper such as zeckensack's Glide wrapper, improves graphics quality and font rendering errors.
Extract from the archive and use. I advise you to install nGlide in addition.