Mika, the blur algorithm uses a fast recursive filter (algorithm in linear time, independent on the blur amplitude). This is not an explicit discrete convolution. See : Recursively implementating the Gaussian and its derivatives - Inria
We have both Deriche and VanVliet recursive filters available in G’MIC, corresponding to slightly different smoothing kernels. Google the names to get the corresponding papers.
You can also quickly simulate a gaussian blur by iterating box blur (command -boxfilter), this is also doable in linear time.