I thought that was the diameter? So you times by pi, right? To get the circumference, and therefore the distance travelled by one turn of the cranks?
No, gear inches are just the equivalent wheel diameter, carried over from penny farthings.
On most bikes you quote the real figure based on the wheel size, track bikes use a slightly different method and quote it for a 27″ wheel for historical reasons.
If you measure how far the wheel travels per crank revolution, that’s usually given in metric units to avoid confusion.