The most basic of all filters in many graphics editing software, it blurs adjacent pixels to a varying degree to create a very smooth image. Compared to real camera out of focus blur it looks very artificial. Some teenage girls use Gaussian Blur on their Myspace angled portrait photos to hide their ugly pores and pimples ending up looking silly, comparable to a man with a plastic mask and a vendetta.
You gotta be kiddin' me! Nobody is going to believe that the Gaussian Blur you just applied is real! You should have used the Lens Blur instead for more realistic look.