It is not directly related to the number of gears -- if you put on a big enough chainring you could achieve higher gears with a few number of gears. It gives a number by which one can describe how many times the wheel will spin for each revolution of the crank. Technically, a gear inch tells you something about the equivalent wheel size on a penny farthing.

http://www.sheldonbrown.com/gloss_g.html#gearinch

http://en.wikipedia.org/wiki/Gear_inches

http://www.sheldonbrown.com/gears/