Equivalence of Definitions of Greatest Common Divisor

$(1)$ implies $(2)$
Let $\gcd \set{a, b}$ be the greatest common divisor of integers $a$ and $b$ by definition 1.

Then by definition:
 * $\gcd \set{a, b}$ is the largest $d \in \Z_{>0}$ such that $d \divides a$ and $d \divides b$.

From Common Divisor Divides GCD:
 * $c \divides a \land c \divides b \implies c \divides d$

Thus $\gcd \set{a, b}$ is the greatest common divisor of integers $a$ and $b$ by definition 2.

$(2)$ implies $(1)$
Let $\gcd \set{a, b}$ be the greatest common divisor of integers $a$ and $b$ by definition 2.

Then by definition $\gcd \set{a, b}$ is the (strictly) positive integer $d \in \Z_{>0}$ such that:
 * $(1): \quad d \divides a \land d \divides b$
 * $(2): \quad c \divides a \land c \divides b \implies c \divides d$

From $d \divides a \land d \divides b$, we see that $d$ is a common divisor of $a$ and $b$.

From $c \divides a \land c \divides b$, we see that $c$ is also a common divisor of $a$ and $b$.

Also, we have that $c \divides d$.

From Absolute Value of Integer is not less than Divisors, we see that (in the domain of $\Z_{>0}$):
 * $c \divides d \implies c \le d$

Thus, whatever $c$ may be, it is no larger than $d$.

Therefore, $d$ must be the greatest of all the common divisors of $a$ and $b$.

Thus $\gcd \set{a, b}$ is the greatest common divisor of integers $a$ and $b$ by definition 1.