I don't work with Gimp but I googeled and found the solution (but only in German language

http://www.nofactory.eu/scripts/gimp-tr ... ilder-png/
It is the way to make a 10x10 pix pic (colour to transparenc) and use this as a layer and then you can take it as a paint brush.
So please google the question in your language and you will find it

Werner