Painting the brick would equal maintenance (which might turn some buyers off), and you risk moisture problems behind the brick. Brick is porous and is meant to breathe and dry out from the "house side" out. Paint inhibits that process.

That said, it is your house and you can do what you want with it. If I were in your shoes, I would paint the siding the gray color that you like, and see how it looks with the brick before proceeding with painting any brick. You might like it and/or it might be enough of a change. Once you paint the brick, there is no going back.

Consider staining the brick instead.