Gear inches is merely a gear ratio like 52/13 = 4 times 27, which equals 108. It has nothing to do with how far the bike travels for each revolution of the tire. As others noted, you must know the true diameter of the tire and multiply by 3.14 to get the distance per revolution. It's really a rough equivalent of 100% being the top gear and the rest a percentage of the largest gear ratio.
A typical road tire has a circumference around 83 inches. With a gear ratio of 4/1 a bike will travel 83x4=332 inches per crank revolution.