Let's consider all $$$Ai + Bj > 0$$$ and take the smallest of them $$$d = min(Ai + Bj) = Ax + By$$$.

Let $$$r$$$ be a remainder of $$$A$$$ divided by $$$d$$$: $$$A = dA_1 + r$$$, $$$d > r \ge 0$$$.

Then $$$r = A - dA_1 = A - (Ax + By)A_1 = A(1 - x) - B(A_1y)$$$, so $$$r$$$ can also be represented as $$$Ai + Bj$$$, or $$$r = 0$$$. The former means that $$$r \ge d$$$ (as $$$d$$$ is minimal such number), which leads to contradiction. Then $$$r = 0$$$, meaning that $$$d$$$ is a divisor of $$$A$$$. The same logic applies to $$$B$$$, so $$$d$$$ is a common divisor of $$$A$$$ and $$$B$$$.

Why is it the greatest? Because if $$$g = gcd(A, B)$$$, then $$$d = Ax + By$$$ $$$\vdots$$$ $$$g$$$, implying $$$d \ge g$$$.