MyPaint can’t import GIMP brushes, and it doesn’t really have “arrow brushes”. Because the engine is totally procedural, you have to figure out how to manipulate the settings to get the shape you need. Take a look at the “leaves” brush in the experimental brush group. It uses “Stroke” to change the size of the dabs in a way that changes over the course of the line as you draw. I tweaked that brush a bit and came up with this. It’s not super crisp, but might get the job done. IMO gritty procedural brushes have some charm in their defects 
You can download it here:
https://drive.google.com/open?id=0B2Rzl_WOUDRiVEVwcG1qSEk4Mms
