Basically a certain gear-set is a torque-multiplier. Much like how pulleys act in a rope system, allowing the same force to do greater work on an object(such as a heavy item).
What a "3.73" or "4.10" gear means is that it takes the drivshaft 3.73turns or 4.10turns respectively in order to make the rear tires turn one complete rotation. The more turns the driveshaft makes per tire revolution the less power the engine has to put out to do the same amount of work. Therefore even though the power output of the engine hasn't changed, the mere fact it uses less power makes it seem like the engine produces more power because it uses it more efficiently. Therefore you get quicker acceleration.
2016 Comp Orange GT (current)
Aurora - 2002 Boosted GT! (Sold, sadly)