This is the unique size used on some older GT bikes. In a classic Dumb Move, GT introduced a completely un-necessary new "in between" tire size. The current owners of GT have chosen to ignore the needs of those who bought GT bikes, and these tires are no longer available.
For years, this size was mistakenly designated as 587 mm ISO size. The actual ISO size is 583 mm. Most likely, a 584 mm (650B/27.5) tire will fit in place.