Two methods to create a new brush easily:
First, the “superfast” method. You have an image area you want make a brush from it, to be used with a tool like pencil, airbrush... Select it with the rectangular (or elliptical) select tool, then do a of this selection and immediately you can see this copy in the first position of the Brush Dialog, and its name is “Clipboard”. It is immediately usable.
This brush is temporary: it disappears when you close GIMP. With GIMP-2.10, you can make it permanent by clicking on the
at the bottom of the brush panel.The second method is more elaborate.
Do Color Space in Gray Level and Fill with: white.
→ with, for example, a width and a length of 35 pixels and in the advanced options aZoom on this new image to enlarge it and draw on it with a black pencil.
Save it with a .gbr extension in the directory /home/name_of_user/.config/GIMP/2.10/brushes/.
In the Brushes dialog window, click on the button Refresh brushes .
And your marvellous brush appears right in the middle of the other brushes. You can use it immediately, without starting GIMP again.