Others have explained how to calculate, so I'll add a note...
Gear inches are the equivalent diameter of direct a drive wheel. In other words, imagine a scenario where each rotation of your pedals resulted in exactly one rotation of the that wheel...
A 24 gear inch would be equivalent of a 24 inch diameter wheel (relatively small)
A 96 gear inch would be equivalent of an eight foot high wheel (rather big even by high wheeler standards).
One rotation of that eight-foot tall wheel will move you four times as far as one rotation of that two-foot tall wheel.