You wrote it yourself: the gcd is the smallest positive linear combination. Smallest positive linear combination is shorthand for smallest positive number which is a linear combination. It is true that $0$ is a linear combination of $12$ and $6$ with integer coefficients, but $0$ is not positive.
The proof is not difficult, but it is somewhat lengthy. We give full detail below.
Let $e$ be the smallest positive linear combination $as+bt$ of $a$ and $b$, where $s$ and $t$ are integers. Suppose in particular that $e=ax+by$.
Let $d=\gcd(a,b)$. Then $d$ divides $a$ and $b$, so it divides $ax+by$. Thus $d$ divides $e$, and therefore in particular $d\le e$.
We show that in fact $e$ is a common divisor of $a$ and $b$, which will imply that $e\le d$. That, together with our earlier $d\le e$, will imply that $d=e$.
So it remains to show that $e$ divides $a$ and $e$ divides $b$. We show that $e$ divides $a$. The proof that $e$ divides $b$ is essentially the same.
Suppose to the contrary that $e$ does not divide $a$. Then when we try to divide $a$ by $e$, we get a positive remainder. More precisely,
where $0\lt r\lt e$. Then
This means that $r$ is a linear combination of $a$ and $b$, and is positive and less than $e$. This contradicts the fact that $e$ is the smallest positive linear combination of $a$ and $b$.