That's odd. I've always thought the way to simulate day and night was done by gradually changing colours on the textures according to a predetermined script and just removing the "overhead lights" if there ever was any. I can't imagine Gothic/Gothic 2 has two of every texture in memory since that game never loads any data in-game, and it's world is huge, varied and very detailed (meaning lots and lots of textures).
It seems, though, as the sun in Gothic/Gothic 2 is one large lightsource that's slowly crawling across the sky, since if the sun passes behind a cloud, that cloud actually casts a huge, real shadow on the ground. Which looks great, by the way.