I would like to create a new brush for my game (NOT a static mesh)

I want to create a new brush to use in my game so that I could still edit it with subtractive brushes.

I’m not sure I completely understand what you mean, but are you asking how you can make a new geometry brush? If you are trying to make a completely new geometry brush you are going to have to edit the engine code where they make the geometry brushes. Or you could just make do with one of the defaults.

Yeah, I’m trying to make a new geometry brush. Do you know any tutorials that could help me learn how?