I have dabbled in making custom brushes for Gimp as well and released one of my creations. Here you can find the download link to it.
Random Bananas – a brush that creates random bananas in six (6) different cartoony, flat-colored bananas in seven (7) different stages, meaning that there are 42 different bananas in this brush. I made it for my illustration for DamonWakes‘s FFM omnibus way back in 2019. It’s distribured under the CC BY-SA 4.0 license. You can download it via this link.