Gaussian blur effect creates strange lines?

[youllfindtheway]

So I wanted to make a background with some shading, I used gaussian blur at 250, and some lines appeared, as if in some low res photo. Sometimes this get really bad.
I wonder what causes this, is there a way to use a lot of blur without getting these lines?

Thanks,

Here's an example:

 

[JgraphNet]

The "effect" you get is known as Banding and Gaussian Blur unfortunately produce these artefacts at high settings. One way to reduce them is to work with 16-bit images and another is to put a small ammount of Noise on the layer(-s) with Gaussian Blur to kind of hide the banding. Sometimes it helps to put the blur on in several steps a bit at the time, but I wouldn't bet my money on it. Try other blurs as well.