The GCD of given numbers is 3.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}177 =& 3\cdot59\\[8pt]137688 =& 2\cdot2\cdot2\cdot3\cdot5737\\[8pt]\end{aligned}$$(view steps on how to factor 177 and 137688. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}177 =& \color{blue}{\boxed{3}}\cdot59\\[8pt]137688 =& 2\cdot2\cdot2\cdot\color{blue}{\boxed{3}}\cdot5737\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 3 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.