To query all available sites, issue a query to the sites field of the content root:

{
  content {
    sites {
      id
      name
      locale
      root {
        ...Reference
      }
      crops {
        name
        aspectRatio {
          width
          height
        }
        sizes {
          width
          height
        }
      }
    }
  }
}

fragment ContentInfo on Content_ {
  name
  creationDate
}

fragment Reference on CMLinkable {
  ...ContentInfo
  title
  segment
  link {
    id
    type
  }
}