Hope you have a good reason for such denormalization - otherwise it would be better to save the list to a child table.

Otherwise, querying would be complicated. Like:

  def days = [2,8,11]
  // note to check for empty days
  Schedule.withCriteria {
    days.each { day ->
      or {
        like('username', "$day,%") // starts with "$day"
        like('username', "%,$day,%")
        like('username', "%,$day") // ends with "$day"
      }
    }
  }