1. GCD is the largest positive integer(with one exception) that divides each of the integers in a given set. 2. Even if one or more input numbers are negative, output is always positive. 3. Both inputs/output are integers. 4. gcd(a,0) = |a| 5. gcd(0,0) = 0, this is defined like this. Though gcd(0,0) can be any integer. This is the only case where gcd is not positive. It is 0.