The GCD of given numbers is 7.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}623 =& 7\cdot89\\[8pt]406 =& 2\cdot7\cdot29\\[8pt]\end{aligned}$$(view steps on how to factor 623 and 406. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}623 =& \color{blue}{\boxed{7}}\cdot89\\[8pt]406 =& 2\cdot\color{blue}{\boxed{7}}\cdot29\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 7 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.