GCD[, , ... ] gives the greatest common divisor of the integers .
Integer mathematical function (see Section A.3.10).
GCD[, ... ] gives the integer factors common to all the .
GCD also works with rational numbers; GCD[, , ... ] gives the greatest rational number r for which all the /r are integers.
GCD has attributes Flat and Orderless.
See The Mathematica Book: Section 3.2.4.
See also: PolynomialGCD, Rational, LCM, ExtendedGCD.