Compared to GS, Xash3D applies different brightness-gamma curve to lightmaps and doesn't apply it to textures at all. This patch fixes it.
Download and extract xash.dll into Xash3D root directory.
Launch Xash3D with "-console" parameter, and then in console set gamma 2.5, brightness 1.2
Picture should look just like it was in Goldsource.
In common.h set texgamma to 0.655 (found experimentally), in Image_SetPalette wrap 3 texture channels into TextureToGamma()
In BuildGammaTable() set g3 = 0.125 unconditionally.
Xash3D build 4529