gcd(a, 0) = a for any a > 0.
gcd(a, b) = gcd(b, a). Order does not matter.
gcd(a, b) = gcd(a - b, b) if a > b. This is a slower version of the Euclidean algorithm.
If d = gcd(a, b), then gcd(a/d, b/d) = 1. Dividing by GCD makes numbers coprime.
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$ curl repovive.com/roadmaps/math-fundamentals/gcd-lcm/gcd-properties
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