Gear inches is how far the bike travels with every 360 degree revolution of the crank. It varies depending on what the combinations of front and rear gears are in use. In use it's just a method of comparing the gearing possibilities.
This is the most useful calculator I've found, even better then Sheldon's. It'll do gear inches, rollout, gain ration and speed. Easy to use.
https://mike-sherman.github.io/shift/