Sorry, I'm not feeling the gray fabric. It is a different direction from all the other fabrics. They gray in the quilt blocks are a stand out already and look great. Don't add competition to them. I'm still a newbie so just offering another opinion to the mix. Of course you can do anything that strikes your fancy and it will be right.
Do you have any of the colorful fabrics used in the quilt? Try those next to the quilt. Have fun and play.