This filter, available through Filters/Blur/Gaussian Blur, gives a more naturalistic look to the transitions between adjacent colors when blurring. It achieves this by blurring unevenly, as is the case in real life, according to the distribution calculated using the Gaussian Bell Curve. Essentially this filter reduces detail in an image, while trying to avoid the artificial look of too smooth a blur across the image.
In the "Gaussian Blur" dialog set the following:
Use the Blur Radius slider (or type in the value directly) to set the radius of blur, between 1 and 100. This controls the amount of blurring by using the blur radius, as a measure from the center of each pixel, for averaging of color values. The greater the blur radius the greater the overall blurring of the image.
The Blur Horizontally and Blur Vertically checkboxes are both checked by default. Click to uncheck either or both of these. As their names indicate they determine the direction of the blur. You would normally want to blur in both directions. Note that unchecking both boxes makes it impossible to blur at all.
All your settings will immediately appear in the Preview box.
See: