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.
Here you can set the blur intensity. By altering the ratio of horizontal to vertical blur, you can give the effect of a motion blur. You can choose the unit with the drop list.
IIR: IIR stands for ΓÇ£Infinite Impulse ResponseΓÇ¥.This blur works best for large radius values and for images which are not computer generated.
RLE: RLE stands for run-length encoding. RLE Gaussian Blur is best used on computer-generated images or those with large areas of constant intensity.