GCD of two huge numbers - IGGI

http://appinventor.mit.edu/explore/master-trainers.html#nguyen

This app is created using MIT App Inventor to demonstrate its ability in CS courses, especially Algorithms.

In mathematics, the greatest common divisor (gcd) of two or more integers, when at least one of them is not zero, is the largest positive integer that divides the numbers without a remainder. For example, the GCD of 8 and 12 is 4.
https://en.wikipedia.org/wiki/Greatest_common_divisor