The grey bands are just a range of lightness values so that the gradient is ‘sampled’ over a wide range of its spread of colours.
The GIMP tutorial is probably compatible with krita but you 'll need to get familiar with the controls and UI in krita. I made it quickly just to see how it all worked.
I’d recommend that you experiment with different .png brushtip images, in greyscale but made in an 8-bit RGB profile image.
If you have an image that is a gretyscale colour profile, you can convert its colour profile to RGB and export it as .png then import it into resources as a brushtip.
Try them with different gradients selected.
Here’s some playing I did using gradients and a gradient map brush:
Glittery!