GCD is an acronym for Greatest Common divisor, which is the largest positive integer of a set of numbers. The GCD can be used for creation of formulae, and in some cases forms a natural working model for efficient calculation using a specific integer as a working base. If you have X number of units in different groupings, and require a form of division which will divide each, the gcd gives you a simplified common divisor, creating a working base for each set.
Examples of GCD:
Set: 144, 24, 36, 84, 156 GCD is 12|
Set: 189, 27, 96, 126, 87 GCD is 3