Gear inches is actually the distance you would travel with one crank rotation divided by pi.
ie: for a 27" diameter wheel, a 45 tooth chain ring, and a 15 tooth cog . . .
Circumfrence = 27" x pi
Ratio = 3:1
Distance traveled in one rotation = 27" x pi x 3 = 81" x pi
Gear inches = 81"
It is intended to equate the diameter of the old penny farthing to modern bikes.