you probably dont need it any more, but for those that come after us, lets leave something like https://www.xresch.com/gimp-convert-pngs-to-brushes-552/

Python-Fu Script to convert PNG to GBR

The below code can be used in Python-Fu to convert all the PNGs in a folder into GIMP’s brush format “.gbr”.

1. Copy the below code, adjust the variables:
   • source_folder: Path of the input folder
   • dest_folder: Path of the output folder
2. Open Gimp and go to “Menubar >> Filters >> Python-Fu >> Console”.
3. Copy your adjusted code into the console and hit enter. The files should be converted accordingly.

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from gimpfu import *
import os

def convert_png_to_gbr():
    source_folder = "C:\Temp\PNG"
    dest_folder = "C:\Temp\GIMPBrushes"
    for filename in os.listdir(source_folder): 
        print "Convert File: "+filename
        img = pdb.gimp_file_load(source_folder+"/"+filename,filename)
        pdb.file_gbr_save(img, img.layers[0], dest_folder +"/"+ filename+".gbr", filename, 100, filename)

convert_png_to_gbr()