Gibraltar isn't that bad. It was the first mission I finished in SH3 when I bought it back in 2010. Day or night doesn't matter much. All one has to do is plot a course along the African coast, go as deep as you are able and go silent. It takes several hours and you will deplete a bunch of your air, but it is manageable. I would not want to try Gibraltar on the surface at all. Radar ignores fog and darkness.
