Dry brush is just a name of a brush in PS, there are a few of them. It's basically a brush with a texture additive so that the brush footprint isn't just plain ole round or square. As for dual brush, that's another PS thing where 2 brushes are combined to give a different footprint. A color-changing brush in PS uses the two current colors selected and each iteration of the footprint is a color along a gradation between the two colors, say like black and white are the two colors then your brushstroke will vary the colors along that gradation using grays. I do not know if Gimp has similar capabilities.