The GCD of given numbers is 11.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}1001 =& 7\cdot11\cdot13\\[8pt]121 =& 11\cdot11\\[8pt]\end{aligned}$$(view steps on how to factor 1001 and 121. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}1001 =& 7\cdot\color{blue}{\boxed{11}}\cdot13\\[8pt]121 =& \color{blue}{\boxed{11}}\cdot11\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 11 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.