Excerpt from wiki:
In mathematics, the greatest common divisor (gcd), also known as the greatest common denominator, greatest common factor (gcf), or highest common factor (hcf), of two or more non-zero integers.
Some recursive algo I remember off the top:
.if b = 0 then return a
.else gcd(a, b mod a)
the modulo is operated *mainly* upon integers(see: fmod).