Look ma, no plugins: the "Freaky details" effects, using of-the-shelf Gimp 2.10

[Ofnuts]

Gimp 3.x update:

In Gimp 3.x you apply the blur has an un-merged filter. Then:

• you select the blurred layer, and click fx and the filter to edit the blur settings (this should open the dialog for the filter)
  
•  while the dialog is up, select the group in the layers list
  

You can now simultaneously tweak the opacity of the group and the amount of blur to fine-tune the result interactively.