From left to right: original, filter applied.
You can find this filter in the image menu under
The IIR Gaussian Blur plug-in acts on each pixel of the active layer or
selection, setting its Value to the average of all pixel Values present
in a radius defined in the dialog. A higher Value will produce a higher
amount of blur. The blur can be set to act in one direction more than
the other by clicking the Chain Button so that it is broken, and
altering the radius. GIMP supports two implementations of Gaussian Blur:
IIR G.B. and RLE G.B. They both produce the same results, but each one
can be faster in some cases.