His books are enjoyable, but they don't do much to challenge the reader's 'gray matter'. [='intelligence']
His hair is 'graying'. = He has 'graying' hair.
What will you do when you are old and 'gray'?
(..) Your friend George is going to sing like a canary. And so are you. And once you've sung, Charlie, my boy, you're gonna take your place on that long, gray line of American manhood. And you will be through. I'd like to disagree with you, Colonel. You're in no position to disagree with me, boy. (..)1992 Scent of a Woman