The GCD of given numbers is 4.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}124 =& 2\cdot2\cdot31\\[8pt]148 =& 2\cdot2\cdot37\\[8pt]\end{aligned}$$(view steps on how to factor 124 and 148. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}124 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{2}}\cdot31\\[8pt]148 =& \color{blue}{\boxed{2}}\cdot\color{red}{\boxed{2}}\cdot37\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 2\cdot2 = 4 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.