I use Clip Studio Paint. The Gaussian Blur filter is at the top bar, under "filter" -> "blur" -> "Gaussian blur". Experiment to see what looks best for you.
I'm glad it helped! If you have Photoshop for finishing work you can also look at lighting and diffusion to go with the rest of the process.
I used Clip Studio Paint, and you can animate in it. I haven't done much animating in it, however, but it can be done and it isn't too complicated.