The Gimp has two different options in how you can perform Gaussian blur: "IIR" and "RLE". What's the difference between them?
From the GIMP docs:
IIR stands for “infinite impulse response”. This blur works best for large radius values and for images which are not computer generated.
RLE stands for “run-length encoding”. RLE Gaussian Blur is best used on computer-generated images or those with large areas of constant intensity.
They both produce the same results, but each one can be faster in some cases.
So the short answer is that IIR is optimised for photographs, and RLE is optimised for images with areas of flat colour.
The claim seems to be that the only difference is the time taken to calculate the blur, and that the end results are identical. I haven't tested this, so if you need to rely on that claim you should test both and check for any differences in the results.