Gimp has been the best free alternative to photoshop, and it matches almost 90% of the features. and like you, all know photoshop has a great brush system, especially if you do photo manipulations, and surprisingly you can do the same in gimp, gimp has a different brush file system, but you can also install photoshop brushes in gimp.
GIMP Brushes for Photo Manipulation
How to install Photoshop Brush in Gimp
Step 1: Open Gimp
Step 2: Go to Edit - preferences
Step 3: Go to Folder and then Bruises
Step 4: Click on Show file location on the file manager.
Step 5: Paste your Bruises file into the Bruises folder.
Hair Brushes
There are many times you had to work with hairs and furs, and hairbrushes can ease a lot of works for you whenever you are working with people and animals.
Grass Brushes
Grass Brushes is also one of the most important bruises for photo manipulation because many times you have to work with the environment and grass-like blending any images with grassy land.
Smoke Brushes
Smoke Brushes can be used in many places like working with fire-related images and many cinematics images.
Fog Brushes
Fog Brushes can give you a more foggy and environmental feel in your composites, and sometimes they can create distance in your composites.
Lightning Brushes
Lightning Brushes is not as important as the other ones, but it can also help in many composites.